CHAPTER 34: The Plant Body

I. Introduction

·         The oldest known plant is a bristlecone pine. It is more than 4,900 years old

·         Like animals, plants must acquire essential compounds to live, including water, carbon dioxide, and minerals.

·         Unlike most animals, most plants cannot move. By growing, plants accomplish some of the same things that animals achieve through motion.

·         Plants can respond to their environment via chemically mediated signals.

II. Vegetative Organs of the Flowering Plant Body

·         Flowering plants possess three kinds of vegetative (nonreproductive) organs: roots, stems, and leaves.

·         Most flowering plants belong to one of two major lineages.

·         Monocots are generally narrow-leaved flowering plants such as grasses.

·         Eudicots are broad-leaved flowering plants such as roses.

·         Monocots and eudicots account for 97% of the species of flowering plants.

·         Most of the remaining species (including water lilies and magnolias) are structurally similar to eudicots, but they are not included with the eudicots because the resulting lineage would not be monophyletic.

·         The shoot system of a plant consists of the stems and the leaves, as well as flowers.

·         Leaves are the main sites of photosynthesis in plants.

·         Stems hold and display the leaves to the sun and provide connections for the transport of materials between roots and leaves.

·         A node is the point where a leaf attaches to a stem.

·         Regions of stem between nodes are known as internodes.

·         The root system provides support and nutrition.

Roots anchor the plant and take up water and minerals

·         There are two main types of root system: taproot and fibrous root.

·         Many eudicots have a taproot system: a single, large, deep-growing primary root with smaller lateral roots.

·         Monocots and some eudicots have a fibrous root system composed of numerous thin roots roughly equal in diameter.

·         A fibrous root system holds soil in place very effectively.

·         Some plants have adventitious roots. These roots arise from points along the stem where roots would not usually occur.

Stems bear buds, leaves, and flowers

·         A bud is an embryonic shoot.

·         A stem bears leaves at its nodes, and where each leaf meets the stem, there is a lateral bud.

·         At the tip of each stem or branch, there is an apical bud, which produces the cells for the growth and development of that stem or branch.

·         Some stems are highly modified.

·         A potato is a portion of the plant's stem, and its "eyes" contain lateral buds.

·         The runners of strawberries are horizontal stems.

Leaves are the primary sites of photosynthesis

·         Leaves are well adapted for gathering light.

·         The blade of a leaf is a thin, flat structure attached to the stem by a stalk called a petiole.

·         The petiole holds the leaf at an angle almost perpendicular to the sun.

·         Leaves at different sites on a single plant can be shaped differently.

·         Most species of plant bear leaves of a particular broadly defined type.

·         A simple leaf has a single blade.

·         A compound leaf has multiple blades (or leaflets) arranged along an axis or radiating from a central point.

·         Some plant species have highly modified leaves, such as the thorns of a cactus.

Plant Cells

·         Plant cells have all the organelles common to eukaryotes.

·         Some plant cells have additional distinguishing features including chloroplasts (or other plastids) and vacuoles.

·         Every plant cell is surrounded by a cellulose-containing cell wall.

Cell walls may be complex in structure

·         Cell wall formation is the final step when plant cells divide.

·         The daughter cells secrete a glue that constitutes the middle lamella, which forms a layer between them.

·         Next, each daughter cell secretes cellulose and other polysaccharides to form a primary wall.

·         Finally, once cell expansion stops, some plant cells may deposit more polysaccharides, sometimes impregnated with material such as lignin (in wood) or suberin (a complex lipid in cork).

·         Additional polysaccharides added after cell expansion are referred to as the secondary wall.

·         Plasmodesmata are pore-like structures that pass through primary cell walls, allowing substances to move freely from cell to cell without crossing the plasma membrane.

·         A plasmodesma is wide enough that portions of the endoplasmic reticulum extend between cells.

·         Pits allow water and nutrients to pass between cells that have a secondary wall.

·         Parenchyma cells are alive when they perform their functions

·         Parenchyma cells are the most numerous type of cell in young plants.

·         Parenchyma cells usually have thin walls and large central vacuoles

·         The photosynthetic cells in leaves are parenchyma cells filled with chloroplasts.

·         Some parenchyma cells store lipids or starch.

·         Other parenchyma cells serve as "packing material" and play a vital role in supporting the stem.

Collenchyma cells provide flexible support while alive

·         Collenchyma cells are supporting cells that lay down primary cell walls that are thick in the corners.

·         Collenchyma provides support to leaf petioles, nonwoody stems, and growing organs.

Sclerenchyma cells provide rigid support after they die

·         Sclerenchyma cells are the main supporting cells of a plant.

·         There are two types of sclerenchyma cells: elongated fibers and variously shaped sclereids.

·         Fibers often organize into bundles.

·         Sclereids may pack together very densely.

Xylem transports water from roots to stems and leaves

·         The xylem conducts water from roots to aboveground plant parts.

·         Conducting cells called tracheary elements are the part of xylem that transports water and minerals.

·         Tracheids are evolutionarily more ancient tracheary elements found in gymnosperms.

·         Both tracheary elements and tracheids undergo programmed cell death and do their jobs as empty cells (only the cell walls remain).

·         Vessel elements are the water "pipeline" system in flowering plants.

·         The cells that form vessels are dead and empty like tracheary elements.

·         Vessel elements are generally larger in diameter than tracheids and are laid down end-to-end to form hollow tubes.

·         During the course of angiosperm evolution, vessel elements have become shorter, and their end walls have become less obliquely oriented and obstructed.

Phloem translocates carbohydrates and other nutrients

·         Cells of the phloem are alive when they do their job, unlike those of the xylem.

·         The characteristic cell of the phloem is the sieve tube member.

·         Like vessel elements, the cells of the phloem are arranged end-to-end and form long sieve tubes, which transport carbohydrates and other materials.

·         The plasmodesmata in sieve tube members enlarge as they mature, resulting in end walls that look like sieves.

·         At functional maturity, a sieve tube is filled with sieve tube sap (water, sugars, and other solutes).

·         The sieve tube members have adjacent companion cells.

·         Companion cells retain all their organelles and may, through activities of their nuclei, regulate the performance of the sieve tube members.

Plant Tissues and Tissue Systems

·         A tissue is an organization of cells that work together as a functional unit.

·         Parenchyma cells make up parenchyma tissue, which is a simple tissue (made of one cell type).

·         Xylem and phloem are complex tissues; they are composed of a number of different cell types.

·         Tissues are grouped into tissue systems that extend throughout the body of the plant from organ to organ.

·         There are three plant tissue systems: vascular, dermal, and ground.

·         The vascular tissue system includes the xylem and phloem; it is the conductive or "plumbing" system of the plant.

·         The phloem transports carbohydrates from sites of production (sources such as leaves) to sites of utilization (sinks) elsewhere in the plant.

·         The xylem distributes water and mineral ions taken up by the roots to the stem and leaves.

·         The dermal tissue system is the outer covering of the plant.

·         All parts of the young plant body are covered by an epidermis, which is a single or multiple layer of cells.

·         The epidermis contains epidermal cells and other specialized cells such as guard cells.

·         The shoot epidermis secretes a layer of wax-covered cutin, the cuticle, which helps retard water loss from stems and leaves.

·         The ground tissue system makes up the rest of a plant and consists primarily of parenchyma tissue.

·         Ground tissue functions primarily in storage, support, photosynthesis, and the production of defensive and attractive substances.

Forming the Plant Body

·         The plant establishes its basic body plan in early embryonic stages.

·         Two patterns contribute to the plant body plan.

·         The apical–basal pattern is the arrangement of cell and tissues along the main axis from root to shoot.

·         The radial pattern is the concentric arrangement of tissue systems.

Plants and animals develop differently

·         The growing stem of a plant consists of modules or units, laid down one after another.

·         Each unit consists of a node with its attached leaf or leaves, the internode below that node, and the later bud or buds at the base of that internode.

·         New units are formed as long as the stem continues to grow.

·         Each branch of a plant may be thought of as a unit that is in some ways independent of the other branches.

·         Leaves are units of another sort, produced in fresh batches to take over the daily function of gathering energy for the plant.

·         Root systems are also branching structures.

·         All parts of the animal body grow as an individual develops from embryonic stages but cease to grow once adulthood is reached (determinate growth).

·         In plants, the growth of roots and stems is indeterminate and is generated from specific regions of active cell division.

·         The localized regions of cell division in plants are called meristems.

·         The cells of meristematic tissues are analogous to the stems cells found in animals.

A hierarchy of meristems generates a plant's body

·         There are two types of meristems.

·         Apical meristems give rise to the primary plant body, which is the entire body of many plants.

·         Lateral meristems give rise to the secondary plant body.

·         The stems and roots of some plants form wood and become thick; it is the lateral meristems that give rise to the tissues responsible for this thickening.

·         Apical meristems:

·         Apical meristems are located at the tips of roots and stems and in buds.

·         Shoot apical meristems supply the cells that extend stems and branches.

·         Root apical meristems supply the cells that extend roots.

·         Shoot and root apical meristems give rise to three types of cylindrical primary meristems that produce the primary tissues of the plant body: protoderm, ground meristem, and procambium.

·         Apical meristems are responsible for primary growth, which leads to elongation and organ formation.

·         Lateral meristems:

·         Some roots and stems develop a secondary body (commonly referred to as wood and bark).

·         Secondary body tissues are derived from two lateral meristems: vascular cambium and cork cambium.

·         Vascular cambium is a cylindrical tissue consisting of vertically elongated cells that divide frequently.

·         The cork cambium produces protective cells that protect the outermost layers of the stem from water loss and microorganisms.

·         The layer of growth of the cork cambium is called the periderm.

·         Growth in the diameter of the stems and roots, produced by vascular and cork cambia, is called secondary growth.

·         Wood is secondary xylem.

·         Bark is everything external to the vascular cambium.

The root apical meristem gives rise to the root cap and the primary meristems

·         The root apical meristem produces all the cells that contribute to growth in the length of the root.

·         The root cap protects the delicate growing area of the root as it pushes through the soil.

·         The root cap also detects the pull of gravity and controls the downward growth of roots.

·         Tissues of the root are divided into three zones: cell division, cell elongation, and cell differentiation.

·         The products of the root's primary meristems become root tissues.

·         Root hairs are long, flattened epidermal cells that increase the root’s surface area.

·         The cortex is directly internal to the root epidermis and often functions in food storage.

·         Endodermis is the next root structure moving in from the cortex. It contains suberin, which makes the cells waterproof.

·         The stele is the vascular component of the root, which includes xylem, phloem, and pericycle tissues.

·         The pericycle consists of one or more layers of undifferentiated cells.

·         In monocots, a region of parenchyma cells called the pith lies in the center of the root.

The products of the stem's primary meristems become stem tissues

·         The shoot apical meristem forms three primary meristems, which in turn give rise to three tissue systems.

·         Leaves form from lateral buds.

·         Vascular tissue in the stem is arranged in vascular bundles.

·         The eudicot stem also contains pith and cortex storage tissues.

Many stems and roots undergo secondary growth

·         Secondary growth increases the diameter of stems and roots.

·         Secondary growth results from the activity of vascular and cork cambia.

·         Vascular rays connect storage parenchyma to the sieve tubes of the phloem.

·         Only eudicots have a vascular cambium and a cork cambium and thus undergo secondary growth.

·         Wood:

·         Cross sections of most tree trunks in temperate zone forests have annual rings

·         Annual rings form due to differential rates of growth in spring (when water is plentiful) and summer.

·         Wood that is no longer conducting water is known as heartwood.

·         Sapwood is wood that is actively conducting water and minerals in the tree.

·         Bark:

·         As secondary growth continues, expanding vascular tissue stretches and breaks the epidermis and cortex, which ultimately break away and are lost.

·         Cork is produced before the epidermis and cortex break and provides a protective function to the underlying tissue.

·         Phelloderm is cork produced on the inside and outside of the stem or root.

·         Lenticels allow gas exchange when bark forms on a stem or root.

Leaf Anatomy Supports Photosynthesis

·         Leaf anatomy is adapted to carry out photosynthesis, limit evaporative water loss, and transport the products of photosynthesis to the rest of the plant.

·         The two zones in leaf parenchyma that photosynthesize are called mesophyll.

·         Veins supply mesophyll cells with water and minerals, and they transport the products of photosynthesis to the rest of the plant.

·         The epidermis of the leaf is the outermost cell layer, which is covered by a waxy cuticle. The epidermis functions to keep water and photosynthetic products in the leaf.

·         Guard cells allow controlled gas exchange through pores in the leaf called stomata.

·         Leaves receive water and mineral nutrients from the roots by way of the stem and veins. In return, the leaves export products of photosynthesis (providing energy) to the rest of the plant.

 

CHAPTER 35 Transport in Plants

Uptake and Transport of Water and Minerals

·         Plants need water for photosynthesis, transporting solutes, temperature control, and developing the internal pressure that supports the plant body.

·         Water enters the plant through osmosis, but the uptake of minerals requires transport proteins.

Water moves through a membrane by osmosis

·         Osmotic potential or solute potential determines the direction of water movement across a membrane.

·         Dissolved solutes have the effect of lowering the concentration of water.

·         The more the solute concentration, the more negative the solute potential (ys), and the more the tendency of water to move toward it from a lower solute concentration.

·         The tendency of a solution to take up water from pure water across a membrane is called water potential, represented by y.

·         For osmosis to occur, a membrane must be semipermeable—permeable to water but not to the solutes.

·         Water potential is the sum of solute potential ys and pressure potential yp: y = ys + yp.

·         Plants are surrounded by a rigid cell wall.

·         As water enters the cell, the plasma membrane presses against the cell wall, restricting its expansion.

·         The cell is turgid, which means it has a high pressure potential.

·         Megapascals (MPa) are the units used to measure pressure potential.

·         Atmospheric pressure is about 0.1 MPa, or 14.7 pounds per square inch.

·         Typical pressure in an automobile tire is about 0.2 MPa.

·         Water always moves across a semipermeable membrane toward the region of more negative water potential.

·         For plants, loss of pressure potential causes wilting.

·         "Bulk flow" is the term used for the movement of fluids in plants due to differences in pressure potential.

Uptake of mineral ions requires transport proteins

·         Ions get through biological membranes either by active transport or by opening gates.

·         When the concentration of ions is greater in the soil than in the plant, the plant can uptake ions by facilitated diffusion.

·         If the concentration of ions is lower in the soil than in the plant, however, ion uptake requires active transport.

·         Electrical and chemical gradients both affect the cell's ability to take up ions.

·         Plants have no sodium–potassium pump for active transport.

·         Plants use a proton pump, which uses ATP to move protons out of root cells, often against a proton gradient.

·         The H+ causes the region outside the membrane to become positively charged.

·         A proton gradient is established.

·         The positive charge outside the cell enhances the movement of positively charged ions (such as K+) into the more negatively charged cell interior through membrane channels.

·         A symport protein couples the diffusion of H+ back into the cell (along its electrochemical gradient) to the transport of Cl- into the cell (against its electrochemical gradient).

·         A membrane potential is established with an increase in intracellular negativity.

·         Plant cells have a membrane potential of at least -120 millivolts.

Water and ions pass to the xylem by way of the apoplast and symplast

·         Where bulk flow of water is occurring, dissolved minerals are carried along.

·         When movement is less, minerals move by diffusion.

·         Minerals must be actively transported across certain membranes.

·         The cells at the surface of the root hairs actively transport ions.

·         Water moves into the cells of the root because the root cells have more negative water potential than the soil solution.

·         Water and solutes get to the dermal and ground tissues and into the stele via the apoplast and symplast.

·         The cell walls and intercellular spaces are the apoplast.

·         Water and minerals can flow without crossing membranes.

·         This is unregulated movement.

·         The symplast is the portion of the plant body enclosed by membranes—the continuous cytoplasm of the living cells.

·         Cells are connected to each other via plasmodesmata.

·         The selectively permeable plasma membranes control access to the symplast.

·         The apoplast allows movement up to the endodermis, the innermost layer of the cortex.

·         The Casparian strips—waxy, suberin-containing structures—create a belt that seals and prevents movement around cells of the endodermis into the stele.

·         Water and ions can enter the stele only by entering and passing through the endodermal cytoplasm.

·         Once past the endodermal barrier, water and minerals leave the symplast.

·         Parenchyma cells in the pericycle or xylem help minerals move back to the apoplast.

·         Transfer cells use ATP to move ions from their cytoplasm (part of the symplast) to their cell walls (part of the apoplast).

·         Increasing the ion concentration in the apoplast lowers its water potential, so water then moves into the apoplast by osmosis.

·         Active transport of ions moves the ions directly, and water follows passively.

Aquaporins control the rate, but not the direction, of water movement

·         Aquaporins are membrane channel proteins through which water moves.

·         They influence the permeability of the membranes, which influences rates of movement but not the direction.

Transport of Water and Minerals in the Xylem

Experiments ruled out some early models of transport in the xylem

·         Eduard Strasburger experimented on trees that were 20 meters tall to determine the nature of water transport.

·         He cut trees at the base and placed the cut ends into a bucket of water and poison.

·         Transport continued until the poison reached the leaves.

·         His experiment determined that the roots were not the cause of transport; that cells were not pumping solutions upward; and that leaves were essential to transport.

Root pressure does not account for xylem transport

·         Root pressure was thought by some plant physiologists to be the force for transport of water and minerals.

·         The pressure is due to the higher solute concentration and more negative water potential in the xylem sap than in the soil.

·         Guttation, in which water is forced out through openings in the leaves, is an example of root pressure.

·         The root pressure is at most 0.1 to 0.2 MPa, however, which is inadequate to account for the ascent of fluids to tree heights.

·         Transport is observed also when negative pressure occurs in xylem.

The transpiration–cohesion–tension mechanism accounts for xylem transport

·         Water is pulled up by transpiration.

·         Transpiration is the evaporation of water from leaves generating a pulling force.

·         Hydrogen bonding between water molecules provides cohesion.

·         The xylem transport system involves transpiration, cohesion, and tension.

·         Tension is created by diffusion of water vapor from the leaves via the stomata.

·         Water vapor diffuses out of the leaf because the concentration of water vapor is higher inside the leaf than outside.

·         As water vapor leaves the leaf, more water evaporates from the moist cell walls of the mesophyll cells.

·         The film of water surrounding the mesophyll cell wall shrinks, and the surface of the film curves where the water retreats into the pores.

·         The surface tension of the curved surfaces generates a negative pressure potential (a pull) in the film.

·         This tension is the force that pulls water all the way up from the roots.

·         The negative pressure and the cohesion of water molecules move water and solutes from vessels or tracheids in xylem.

·         Narrowness of transport tubes increases the resistance of the water column to breakage.

·         The column is maintained to the height of the tallest trees, such as a 100-meter redwood.

·         No energy expenditure is required by the plant. The sun drives the movement.

·         Dry air produces the most negative water potential.

·         Mineral ions in the xylem sap rise passively with the solution.

·         Transpiration also contributes to the plant's temperature regulation, cooling plants in hot environments.

A pressure bomb measures tension in the xylem sap

·         Per Scholander measured tension in stems with a pressure bomb.

·         He found that in all plant species in which xylem sap was ascending, he could measure tension.

·         The tension was absent at night.

·         In developing vines, xylem sap was under no tension until leaves formed.

·         The tension is also an individual measure of water loss.

Transpiration and the Stomata

·         Stomata can be closed to restrict water loss.

·         Stomata open to allow CO2 in.

The guard cells control the size of the stomatal opening

·         A blue light wavelength is absorbed by the guard cell plasma membrane and activates the transport of protons.

·         The proton gradient drives the accumulation of potassium ions (K+) in the guard cells.

·         High intracellular potassium in guard cells, created because of protons pumped out of the cells, causes water to diffuse in and turgor pressure to rise.

·         Stomata open when under high turgor.

·         When water supply is low, abscisic acid (a plant hormone) causes stomata to close, conserving water—possibly at the expense of needed CO2.

·         Stomata also close when protons are no longer pumped (K+ and water diffuse out of the stomata).

Antitranspirants decrease water loss

·         Stomata provide admission of CO2 for photosynthesis, but water is lost by transpiration.

·         An antitranspirant reduces water loss from stomata.

·         A good antitranspirant compound is one that reduces water loss without excessively limiting CO2 uptake.

·         Abscisic acid and commercial chemical analogs work but are expensive.

·         The guard cells of a mutant called era have greater sensitivity to abscisic acid. Plants with the gene are more drought resistant.

·         Growers sometimes use polymeric films on leaves to form a barrier to evaporation.

Crassulacean acid metabolism correlates with an inverted stomatal cycle

·         CAM plants live in dry areas or near the oceans.

·         Their stomata are open at night and closed during the day.

·         They capture CO2 in organic acids at night for daytime use.

·         CAM plants lose much less water than non-CAM plants.

Translocation of Substances in the Phloem

·         Substances in phloem move from sources to sinks.

·         A source is a photosynthesizing leaf or a starch-storing root.

·         A sink is a root, flower, or other structure that has inadequate sugar or amino acids.

·         The phloem was proven to be the tissue for organic solute transport by Marcello Malpighi over 300 years ago.

·         Girdled trees formed a swelling above the girdle, and dead bark below.

·         Plant physiologists use an insect, the aphid, to collect sieve tube sap. These tiny insects normally feed by tapping into phloem.

·         Physiologists watch for liquid on the aphid's abdomen. They then freeze the aphid and cut its body away from its stylet, which remains in the sieve tube.

·         Sieve sap flows through the stylet for hours.

·         Chemical analysis reveals its contents.

·         Radioactive tracers can also be used to study flow rates.

·         Bidirectional translocation occurs by means of different (but close) sieve tubes conducting sap in opposite directions.

The pressure flow model appears to account for phloem translocation

·         Two steps in sieve tube flow require energy:

·         Transport of sucrose and other solutes into the sieve tubes (loading) at sources

·         Removing (unloading) solutes where the sieve tubes enter sinks

·         Cells actively transport solutes into sieve tube members. Water then follows and pressure builds.

·         At the sink, solutes are pumped and/or transported into cells, and water follows.

Plasmodesmata and material transfer between cells

·         Many substances move from cell to cell within the symplast through plasmodesmata.

·         The plasmodesmata participate in the loading and unloading of sieve tube members.

·         In sink tissues, the plasmodesmata connect to sieve tube members.

·         As a leaf matures and transitions from sink to source, the number of plasmodesmata declines.

·         Normally, molecules of up to 1,000 daltons can get through plasmodesmata.

 

 

CHAPTER 36: Plant Nutrition

Introduction

·         Plants need nitrogen for production of proteins and amino acids.

·         Plants also need other materials from the environment.

·         Most nutrients come from soil.

The Acquisition of Nutrients

·         All living things need raw materials from the environment.

·         These nutrients include carbon, hydrogen, oxygen, and nitrogen.

·         Carbon comes from photosynthetic organisms or from the CO2 in the air.

·         Hydrogen comes from water.

·         Carbon, oxygen, and hydrogen enter life through photosynthesis.

·         Nitrogen enters living forms first in bacteria, which can convert N2 in air to forms useful to other life forms.

·         The exception is the small amount of nitrogen fixed by lightning, which is deposited by rain or snowfall.

·         Other mineral nutrients are required for life. These include sulfur, phosphorus, potassium, magnesium, and iron.

·         Plants take up most nutrients as dissolved solutes in the water of the soil.

Autotrophs make their own organic compounds

·         Plants are autotrophs. They make their own organic molecules from carbon dioxide, water, and minerals.

·         Heterotrophs require organic compounds as foods. Herbivores and carnivores get their organic molecules from plants.

·         Most autotrophs photosynthesize.

·         A few do not but instead derive energy from chemicals. These are chemosynthesizers.

·         They use ancient, unrenewable sources of energy that exists in reduced inorganic substances.

·         All chemosynthesizers are prokaryotes.

How does a stationary organism find nutrients?

·         Plants are sessile. Their roots mine the soil for new minerals by growing.

·         Growth of stems and leaves helps the plant acquire more solar energy.

Mineral Nutrients Essential to Plants        

·         All except nitrogen are derived ultimately from rock.

·         An element is said to be essential if it is necessary for normal growth and reproduction, it is not replaceable by another element, and the requirement is direct (i.e., not the result of an indirect effect).

·         Macronutrients are found in dried plant matter at 1 gram per kilogram or more.

·         Micronutrients are found at 100 mg per kilogram or less.

Deficiency symptoms reveal inadequate nutrition

·         Nitrogen is often in short supply for plants in nature. The plants typically show no symptom but grow more slowly than they would if more nitrogen were available.

·         Crop plants, which have been bred to grow quickly, show symptoms when nitrogen is low.

·         Yellowing of older leaves, which is called chlorosis, occurs.

·         Inadequate iron also causes chlorosis, but it tends to affect the youngest leaves.

·         The reason for this difference is that nitrogen can be readily translocated in the plant, but iron is more difficult to translocate.

·         Plants with insufficient nitrogen tend to move nitrogen from older leaves to younger leaves to favor their growth.

·         Plants with insufficient iron cannot move it to the younger leaves, where it is needed for chlorophyll synthesis.

Several essential elements fulfill multiple roles

·         Some minerals are involved in structure, some in catalysis.

·         Magnesium is an essential part of chlorophyll and is a cofactor for many enzymes.

·         Phosphate is involved in energy metabolism (ATP), is found in nucleic acids, and is involved in switching the activities of enzymes.

·         Calcium affects the cytoskeleton and functions in the processing of hormonal and environmental cues.

The identification of essential elements

·         An element is essential if the plant fails to complete its life cycle or grows abnormally without it.

·         This was sometimes difficult to determine until very pure chemicals were available.

·         Even touching a plant might provide it with a significant dose of chlorine.

·         New essential elements are rarely reported now; either the list is nearing completing, or (more likely) new techniques are necessary to discover more.

Soils and Plants

·         Soil provides the minerals plants need.

·         It also provides the water, mechanical support, microorganisms, and oxygen for roots.

Soils are complex in structure

·         Soils are complex mixtures of living and nonliving components, including bacteria, fungi, earthworms, and other animals; particles of rock, clay, water, dissolved minerals, and air spaces; and dead organisms.

·         The structure of many soils changes with depth, revealing a soil profile.

·         Most soils have two or more horizons lying on top of each other.

·         Minerals tend to leach or be carried away by flowing water and sink into deeper horizons.

·         Soil scientists recognize three major zones (A, B, and C).

·         Topsoil is the A horizon.

·         Most organic matter is in A, as are roots, earthworms, insects, nematodes, and microorganisms.

·         Agriculture depends on the A horizon.

·         Sand lacks minerals and water but has plenty of air; clay lacks air but has minerals and water.

·         The best topsoils are loams, which are a mixture of clay, sand, and silt.

·         The B horizon is below A and is called subsoil. It is leached material from A but with little or no organic matter.

·         The C horizon is below B and is the parent rock from which soil is derived.

·         Deep-growing roots extend into B but rarely C.

Soils form through the weathering of rock

·         The type of soil in a given area depends on the type of rock from which it forms, climate, landscape features, organisms living on it, and time.

·         Rocks are broken down by mechanical weathering and chemical weathering.

·         Key to soil formation is the formation of clay, which requires chemical weathering, not just mechanical weathering.

Soils are the source of plant nutrition

·         Clay has a net negative charge due to anions that are permanently attached to clay particles.

·         Magnesium, calcium, and many other important minerals have positively charged ions (cations).

·         Cations in solution are attracted to the anions on clay particles.

·         To be accessible to plants, cations must disassociate from the clay.

·         This is accomplished when hydrogen ions are released by roots. Roots also increase local concentrations of carbonic acid (by respiring and releasing CO2 into the soil).

·         Both these ions exchange with the clay to release the bound cation minerals.

·         Clay particles hold the needed minerals until triggered by the plant (and microorganisms) to release them locally.

·         Nitrogen, sulfur, and phosphorus are found in anions. Clay does not hold them, so they readily leach away in rain or down to lower horizons.

Fertilizers and lime are used in agriculture

·         Agricultural soils often require fertilizer because irrigation and rainwater leach minerals from the soil, and the crop harvest removes nutrients taken up by plants.

·         Three elements commonly used in fertilizers are nitrogen, phosphorus, and potassium.

·         N-P-K percentages are often labeled on fertilizer bags. A 5-10-10 has 5% nitrogen, 10% phosphate (P2O5), and 10% potash (K2O) by weight.

·         Such a fertilizer actually has 4.3% phosphorus (because the 10% “P” is shared by phosphorus and oxygen in P2O5), and 8.3% potassium (because the 10% “K” is shared by potassium and oxygen in K2O).

·         Organic fertilizers help improve soil texture.

·         pH is important and influences the availability of nutrients. pH 6.5 is optimal for most crops.

·         Blueberries prefer more acidic soil, around pH 4.

·         Calcium carbonate, calcium hydroxide, or magnesium carbonate can be added to raise pH, if soil is too acidic.

·         If soil is not acidic enough, sulfur can be added. Bacteria convert it to sulfuric acid.

·         Plants take up more copper, iron, and manganese when they are applied as a foliar (leaf) spray.

Plants affect soils

·         Plant litter, such as dead leaves, break down to form humus.

·         Soil bacteria and fungi produce humus by breaking down plant litter, animal feces, and other organic materials.

·         Humus is rich in minerals, especially nitrogen. Its texture helps to provide roots with oxygen.

·         Plants influence the soil pH.

Nitrogen Fixation

·         Earth's atmosphere is about 80% nitrogen.

·         Plants cannot access atmospheric nitrogen directly; they depend on nitrogen fixers.

·         These bacteria convert N2 to ammonia.

Nitrogen fixers make all other life possible

·         About 170 million metric tons of nitrogen are fixed each year by nitrogen fixers.

·         About 80 million metric tons are fixed by humans, industrially.

·         A smaller amount is produced from lightning, volcanic eruption, and forest fires.

·         These nitrogen products are distributed by rain and snow.

·         Cyanobacteria fix nitrogen in oceans and fresh water.

·         Some soil bacteria fix just what they need and release it when they die.

·         Other nitrogen-fixing bacteria live in close association with plant roots. They release up to 90% of the nitrogen they produce.

·         Rhizobium bacteria fix nitrogen only in close association with the roots of legumes.

·         These bacteria infect plant roots, causing the roots to develop nodules.

·         Farmers coat legume seeds with specific species of Rhizobium.

·         Some cyanobacteria fix nitrogen in association with fungi or with ferns, cycads, or nontracheophytes.

·         Rice farmers increase fixed nitrogen by growing the water fern Azolla in their rice paddies.

·         A species of actinomycete fixes nitrogen in association with root nodules on woody species such as alder and mountain lilacs.

Nitrogenase catalyzes nitrogen fixation

·         Energy is required and is supplied by ATP.

·         A single enzyme called nitrogenase catalyzes the reaction.

·         Nitrogenase is strongly inhibited by O2. Legume nodules provide just enough O2 for respiration but not so much as to inactivate nitrogenase.

Some plants and bacteria work together to fix nitrogen

·         Legume nodules are symbiotic environments where the mutualistic relationship of plants and bacteria occurs.

·         Neither free-living Rhizobium nor uninfected legumes can fix nitrogen.

·         Establishment of this symbiosis requires contributions from both organisms.

·         The root attracts Rhizobium with chemical attractants called flavonoids.

·         These trigger bacterial transcription of nod genes, which are translated into Nod factors.

·         Nod factors influence root growth.

·         A nodule forms, and within it are bacteroids of Rhizobium.

·         Bacteroids are intracellular bacteria surrounded by membranous vesicles.

·         Leghemoglobin is produced by the plant cell. It can make cells of the root look pink. This molecule controls the levels of free oxygen.

Biological nitrogen fixation does not always meet agricultural needs

·         More nitrogen is needed today by agriculture than is available by nitrogen fixation.

·         The Haber process is currently being used by industry, but it is energy intensive.

·         Nitrogen-containing fertilizers require larger energy investments than any other aspect of modern agriculture.

·         Some scientists are trying to genetically engineer plants to fix their own nitrogen.

·         This will require not only inserting genes for nitrogenase, but finding ways to exclude O2, and obtaining strong reducing agents and an energy source.

Plants and bacteria participate in the global nitrogen cycle.

·         Nitrogen enters naturally as ammonia.

·         Nitrasomonas and Nitrosococcus convert this to NO2.

·         Nitrobacter converts NO2 to NO3.

·         Denitrifying bacteria break down NO3 to convert it back to N2.

Sulfur Metabolism

·         All living things need sulfur.

·         It is found in cysteine and methionine and hence in almost all proteins.

·         Plants take up most elements in oxidized form (oxygen is the exception).

·         Sulfate is taken up by plants and then reduced as it is incorporated.

Heterotrophic and Carnivorous Seed Plants

·         A few plants have lost their autotroph status.

·         Some are parasites that live off other plants.

·         Mistletoes are green and photosynthesize, but still parasitize other plants.

·         There are about 450 carnivorous species.

·         These plants grow naturally in acidic environments that are nitrogen poor, and the captured insects help augment nitrogen supplies.

·         The Venus flytraps are examples.

·         Sarrocenia has pitcher-shaped leaves. Insects are attracted by odor. Downward pointing needles prevent escape. Enzymes and bacteria digest the insects.

·         Sundews have leaves with hairs that secrete a clear, sticky liquid.

·         Insects get stuck on the syrup, and the plant digests the insect and absorbs the carbon- and nitrogen-containing products.

 

CHAPTER 37: Plant Growth Regulation

Interacting Factors in Plant Development

·         Plants sense and respond to environmental cues.

·         To do this, plants use receptors, such as photoreceptors that absorb light.

·         Chemical messages, or hormones, mediate the effects of the environmental cues.

·         Enzymes, which are encoded by the plant’s genome, are central to all events in the plant’s life.

Several hormones and photoreceptors regulate plant growth

·         Hormones are regulatory compounds that act at low concentrations at sites distant from where they are produced.

·         Hormones mediate growth and development.

·         Each plant hormone has multiple roles.

·         Photoreceptors are proteins that also regulate aspects of development. (Plant hormones are not proteins.)

·         Photoreceptors respond to light.

Signal transduction pathways mediate hormone and photoreceptor action

·         Plants use signal transduction pathways.

·         Signaling pathways involve a signal and its reception, a signal transduction, and then a cellular response.

·         Plant signal transduction pathways differ from those of animals only in details.

·         For example, plant kinases tend to phosphorylate serine and threonine residues, not tyrosine, as in many animal cells.

From Seed to Death: An Overview of Plant Development

·         As plants develop, environmental cues, photoreceptors, and hormones affect three fundamental processes: cell division, cell expansion, and cell differentiation.

The seed germinates and forms a growing seedling

·         Seeds may be dormant.

·         Seeds are typically 5 to 20% water by weight, which is low relative to most other parts of a plant.

·         When seeds germinate, they imbibe water.

·         The embryo uses reserves by digesting stored polymers into monomers.

The plant flowers and sets fruit

·         Some plants flower when they reach a certain size or age; others flower during certain times of the year.

·         Those that flower based on hours of daylight have means for measuring time—photoreceptors in leaves.

·         When it is time for the plant to flower, a yet-to-be-discovered flowering hormone travels to the future locations of the flower to trigger differentiation.

The plant senesces and dies

·         Perennials continue to grow year after year.

·         Some perennials go into dormancy for part of the year.

·         A hormone called abscisic acid helps maintain this dormancy.

·         In some species, leaves senesce and fall off.

·         This process is regulated by the interactions of auxin and ethylene.

·         Eventually, the whole plant senesces and dies.

Ending Seed Dormancy and Beginning Germination

·         Seed dormancy may last for weeks, months, years, or even centuries.

·         The principal strategies of dormancy are:

·         Exclusion of water and/or oxygen by the seed coat

·         Mechanical restraint of the embryo

·         Chemical inhibition of embryo development

·         Breaking the seed coat can break dormancy.

·         Microorganisms in soil might help soften the seed coat.

·         Fire releases mechanical restraint in some species.

·         Leaching is a way to reduce concentrations of inhibitors.

Seed dormancy affords adaptive advantages

·         Dormancy improves survivability of some plants.

·         Seeds can pass through dry and cold seasons more easily than adult plants can.

·         Some seeds will not germinate, regardless of how they are treated, until a certain minimum amount of time has passed.

·         Those that germinate after fire exposure are ensured first access to areas cleared by fire.

·         Some seeds will not germinate in light. This ensures that they are in the soil.

·         Seeds help counter year-to-year environmental variation. The seed will wait to germinate until favorable conditions exist.

Seed germination begins with the uptake of water

·         The first step in seed germination is imbibition.

·         The interior of the seed has a negative water potential, and water causes the seeds to swell substantially.

·         Cocklebur seeds generate pressures of up to 1,000 atmospheres when they are imbibing.

·         Water uptake activates certain enzymes, and proteins are synthesized.

·         In many seeds, growth results only from the expansion of existing cells, not from cell division.

·         DNA synthesis and cell division occur later, when the radicle begins to grow and poke out of the seed coat.

The embryo must mobilize its reserves

·         Reserves in the endosperm or cotyledon provide energy and materials until the leaves begin photosynthesis.

·         The energy storage molecules include starch, fats, and oils. Storage proteins provide amino acids.

·         Starch yields glucose.

·         Digestion of oils yields glycerol and fatty acids.

·         The embryo secretes gibberellins, a class of plant hormone. The gibberellins trigger changes in the aleurone layer inside the seed coat.

Gibberellins: Regulators from Germination to Fruit Growth

Foolish seedlings led to the discovery of the gibberellins

·         Gibberellins are a large family of closely related compounds. Most are found in plants; some are found in a fungal plant pathogen.

·         They were first studied in the early 1800s.

·         A fungal pathogen caused bakanae, or "foolish seedling" disease, in rice.

·         It was found to be caused by Gibberella fujikuroi, an ascomycete fungus.

·         In 1925, Eiichi Kurosawa demonstrated the role of the fungus in producing a plant growth stimulant.

·         Bernard O. Phinney demonstrated the effect of fungally derived gibberellins on dwarf corn plants.

·         Normal plants were unaffected.

·         Dwarf grew as tall as normal when treated with the hormone.

·         Phinney concluded that plants normally produce gibberellins, but dwarf  plants fail to. Treating dwarf plants with the hormone restores normal growth.

·         More than 80 gibberellins have been identified. Only gibberellin A1 controls stem elongation.

The gibberellins have many effects

·         Seedless grapes normally form smaller fruits than grapes with seeds.

·         Spraying seedless grapes with gibberellins causes them to grow as large as seeded grapes.

·         This is evidence that seeds produce the hormone and that it has a role in fruit production.

·         Biochemical studies have found that developing seeds produce gibberellins, which diffuse out into the immature fruit tissue.

·         Biennial species produce substantially more gibberellins when about to bolt. Applying a gibberellin solution to them can sometimes induce bolting.

·         Gibberellins also cause fruit to grow from unpollinated flowers, promote seed germination in lettuce, and bring spring buds out of winter dormancy.

Auxin Affects Plant Growth and Form

·         Pinching off the apical bud at the top of a plant increases lateral bud growth.

·         If plants are kept indoors, they grow toward a window.

·         These responses are mediated by a plant hormone called auxin, or indoleacetic acid (IAA).

Plant movements led to the discovery of auxin

·         Phototropism is the tendency for plants to grow toward light sources.

·         In the 1800s, Charles Darwin and his son Francis experimented with canary grass seedlings grown in the dark. (See Figure 37.10.)

·         They found that when the top millimeter of the coleoptile is covered, the plant cannot respond to the direction of light. If the region below this is covered, but the tip is exposed, the plant responds to the direction of the light source.

·         Other studies later showed that the substance produced at a cut barrier can diffuse through nonliving barriers like gelatin, but not impermeable barriers like metal.

·         The tip produces a hormone, which moves down to the growing region. The hormone moves down one side of the stem only.

Auxin transport is polar

·         The movement of auxin is polar—it travels in just one direction along a line from apex to base.

·         This movement is not due to gravity, but is biologically controlled.

Auxin carrier proteins move auxin into and out of cells

·         In polar transport, carrier proteins import auxin molecules at one end of a cell and export them out the other end.

·         These carriers contribute to the establishment of the auxin gradient.

Light and gravity affect the direction of plant growth

·         The redistribution of auxin is involved in both phototropism and gravitropism.

·         When light strikes a coleoptile from one side, the side of the stem that gets more auxin is the side away from the light. This side grows a bit more, causing the stem to bend toward the light.

·         Auxin does the same in the dark. Gravitropism is an auxin-mediated effect. The carrier proteins are involved in the response to gravity.

Auxin affects vegetative growth in several ways

·         Auxin initiates root growth in cuttings, stimulates detachment of old leaves, maintains apical dominance, and promotes stem elongation, but inhibits root elongation.

·         Dipping cut surfaces into an auxin solution reprograms the development of some plants to grow profuse roots.

·         Abscission is the result of the breakdown of a specific part of the petiole

·         If the leaf is removed, but the petiole is left, it falls off after a short time.

·         If auxin is supplied, the petiole stays longer.

·         Therefore, declining auxin levels influence timing of abscission.

·         Apical dominance is the tendency for lateral buds to remain dormant. Cutting the growing tips stimulates lateral bud growth, unless auxin is applied to the location where the tip was removed.

·         Auxin stimulates stem elongation but inhibits root elongation.

·         Note, however, that plant growth is regulated more by hormone interactions than by a single hormone.

·         Synthetic auxins have been produced and studied.

·         One of them is lethal to eudicots at concentrations that are harmless to monocots.

·         This auxin has been used as a selective herbicide on lawns—grasses are monocots, and most of the “weeds” in lawns are eudicots.

·         This auxin takes a long time to break down, however, so it pollutes the environment.

Auxin controls the development of some fruits

·         In many species, unfertilized ovaries grow into fruit if treated with auxin or gibberellins.

·         This is useful for production of seedless fruit.

Auxin promotes growth by acting on cell walls

·         Cell walls are key to plant growth.

·         Cell walls are composed of cellulose and a few other components.

·         Cellulose molecules tend to form parallel associations.

·         Microfibrils are composed of about 250 parallel cellulose molecules.

·         Networks of microfibrils make cell walls rigid.

·         Cell growth is driven by the uptake of water and deposit of new cellulose.

·         Auxin loosens the cell wall:

·         Auxin affects the cell wall, increasing its plasticity.

·         Auxin works by causing the release of a cytoplasmic "wall-loosening factor."

·         Proteins called expansins, which are activated by protons (H+), modify hydrogen bonding between polysaccharides in the plant wall.

Plants contain specific auxin receptor proteins

·         The protein receptor ABP1 (Auxin-Binding Protein 1) binds auxin.

·         Other receptors for auxin exist.

Auxin and other hormones evoke differentiation and organ formation

·         Pith cultures are plant cell cultures made from the spongy, innermost tissue of a stem.

·         In culture, pith tissue grows rapidly but is undifferentiated.

·         If a stem tip is inserted into the culture cells, the pith cells differentiate; some become xylem cells.

·         Coconut milk, which is rich in plant hormones, also causes pith cell differentiation.

·         Auxins cause tobacco pith cells to form roots. Cytokinins cause pith cells to form buds. Together, the two hormones make new plants develop.

Cytokinins Are Active from Seed to Senescence

·         Cytokinins aid germination, inhibit stem elongation, stimulate lateral bud growth, and delay leaf senescence.

·         They are derivatives of adenine.

·         Kinetin is a cytokinin that powerfully stimulates cell division in tissue cultures.

·         Kinetin is synthetic; it has never been isolated from plants.

·         Zeatin and isopentenyl adenine, which have the same effect, have been isolated from plants.

·         Cytokinins are mostly produced in roots and move to other parts of the plant.

·         Adding cytokinins and auxins to a growth medium stimulates rapid growth of plant tissues.

·         Cytokinins can cause certain light-requiring seeds to germinate when seeds are kept in constant darkness.

·         Cytokinins usually inhibit the elongation of stems, but they cause lateral swelling.

·         Cytokinins stimulate lateral bud growth.

·         Cytokinins may regulate normal leaf expansion.

·         Cytokinins delay leaf senescence.

Ethylene: A Gaseous Hormone That Promotes Senescence

·         Ethylene is a gas and a plant hormone.

·         Whereas auxin delays leaf abscission, ethylene strongly promotes it.

Ethylene hastens the ripening of fruit

·         The "rotten apple that spoils the barrel" does so because it releases ethylene.

·         Ripening fruit loses chlorophyll, and its cell walls break down.

·         Ethylene promotes this process.

·         Today, commercial shippers and storers of fruit hasten ripening by adding ethylene to storage changers.

·         To delay ripening, scrubbers and absorbents remove ethylene from the atmosphere in fruit storage chambers.

Ethylene affects stems in several ways

·         The apical hook of a eudicot, a protective structure, is maintained by asymmetrical production of ethylene gas.

Abscisic Acid: The Stress Hormone

·         Abscisic acid has multiple effects.

·         It promotes storage in seeds.

·         It is present in high concentrations in dormant buds.

·         It also inhibits stem elongation.

·         It is referred to as a stress hormone because it accumulates when plants are short of water.

·         Some mutant corn plants, called vp mutants, have seeds that germinate while attached to the cob. They are deficient in abscisic acid.

·         Abscisic acid causes stomata to close, which reduces water loss. It causes a release of Ca2+ from the vacuoles of guard cells and allows it into the cell. Potassium channels open, K+ and water leave the cell, and guard cells sag together, closing the stoma.

Hormones in Plant Defenses

·         One of a plant's first responses to fungal or bacterial attack is to release sugar-derived hormones, oligosaccharins.

·         Oligosaccharins are fragments of the plant's own cell wall, which the attacker inadvertently releases by degrading the cell wall.

·         Oligosaccharins act as signals that trigger the plant’s defenses.

·         Auxin also causes release of oligosaccharins.

·         Jasmonates, salicylic acid, and systemin are other hormones that serve as important signals in plant defenses.

Brassinosteroids: "New" Hormones with Multiple Effects

·         Brassinosteroid is a steroid first isolated from the pollen of rape (a member of the mustard family).

·         It stimulates cell elongation, pollen tube elongation, and vascular tissue differentiation, and it inhibits root elongation.

·         Dozens of related compounds have been discovered.

·         An Arabidopsis mutant called det2 has seedlings that grow in the dark as if in the light. Treating with brassinosteroids returns these seedlings to normal growing patterns. Therefore, det2 plants are unable to produce their own brassinosteroids.

·         Joanne Chory found that a mutation of the gene bas-1 in Arabidopsis could be exploited to produce height control in plants.

·         With application of brassinosteroid, the plant grows normally. Without, the plant growth is retarded.

·         Possibly lawn grass or hedge heights could be regulated this way.

Light and Photoreceptors

·         Onset of winter dormancy is controlled by the length of the night.

·         Other environmental cues are light color, intensity, duration, and temperature.

·         Some photoreceptors measure dark cycles. These are called phytochromes.

·         Five phytochromes mediate the effects of red and dim blue light.

·         Three or more blue-light receptors, discovered more recently, mediate the effects of higher-intensity blue light.

Phytochromes mediate the effects of red and far-red light

·         Far-red light reverses the effect of a prior exposure to red light.

·         Far-red is 730 nm wavelength; red wavelength is centered around 660 nm.

·         If seeds are exposed to short cycles of red and far-red light, they respond to just the last one.

·         Phytochromes are bluish-colored photoreceptor proteins.

·         There are two interconvertible forms. Light drives the conversion.

·         Pr absorbs red and then becomes Pfr.

·         Pfr absorbs far-red and then becomes Pr.

·         Pfr has important biological effects, such as initiating germination in certain seeds.

Phytochromes have many effects

·         Seeds that germinate in the dark and form etiolated plants cannot carry out photosynthesis.

·         Exposing the plant to light converts Pr to Pfr, and chlorophyll synthesis begins.

There are multiple phytochromes

·         Arabidopsis has five genes that encode different phytochromes.

·         This diversity has been found throughout the plant kingdom and in algae.

·         Some phytochromes may be involved in daily growth.

·         Phytochromes appear to activate one or more G proteins.

Cryptochromes and phototropin are blue-light receptors

·         Cryptochromes are yellow photoreceptor pigments. They absorb blue and ultraviolet light.

·         These photoreceptors are found in plants and animals.

·         They are found in plant nuclei.

·         Plant scientists have found convincing evidence that the photoreceptor for phototropism is a yellow protein called phototropin.

·         Upon absorbing blue light, phototropin initiates a signal transduction pathway leading to phototropic curvature.

 

CHAPTER 38 Reproduction in Flowering Plants

Many Ways to Reproduce

·         Plants reproduce in many ways.

·         Some reproduce sexually, others both sexually and asexually, and yet others only asexually.

·         Asexual reproduction produces offspring that are genetically identical to their parents.

·         Sexual reproduction produces new genetic combinations.

·         Most agricultural crops reproduce sexually, but many reproduce asexually.

·         Navel oranges are seedless and are cultivated through asexual reproduction.

·         Strawberries can reproduce either sexually or asexually, but in agriculture they are cultivated through asexual reproduction.

Sexual Reproduction

·         Sexual reproduction provides new genetic combinations.

·         Adaptability results from the greater genotypic diversity.

The flower is an angiosperm's device for sexual reproduction

·         Flowers have four groups of organs, all of which are modified leaves: carpels, stamens, petals, and sepals.

·         Carpels are female sex organs, and stamens are male sex organs.

·         A pistil is a structure composed of one or more carpels.

·         The base of the pistil is the ovary, which contains one or more ovules.

·         Each ovule contains a megasporangium.

·         The stalk of the pistil is the style, and the end of the style is the stigma.

·         Each stamen is composed of a filament bearing a two-lobed anther, which consists of four microsporangia fused together.

·         Petals and sepals of many flowers are arranged in whorls (circles) around the carpels and stamens.

·         Together, petals constitute the corolla.

·         Below this, the sepals constitute the calyx.

Flowering plants have microscopic gametophytes

·         Female gametophytes, the megagametophytes, are called embryo sacs and develop in megasporangia.

·         Male gametophytes, the microgametophytes, are called pollen grains and develop in microsporangia.

·         At one end of the megagametophyte are three tiny cells, the egg and the two synergids.

·         At the opposite end are the antipodal cells, which eventually degenerate.

·         In the large central cell are two polar nuclei.

·         The embryo sac is the seven-celled, eight-nucleus structure.

Pollination enables fertilization in the absence of liquid water

·         Gymnosperms and angiosperms can fertilize independent of water.

·         Pollination is sometimes by direct contact of male and female flower parts.

·         Wind and animals carry pollen of other species.

Some plants practice "mate selection"

·         Many plants are incompatible with their own pollen.

·         This rejection promotes outcrossing.

·         The S gene controls self-incompatibility. There are dozens of S alleles. A pollen grain with the same S allele as the flower it lands on fails to develop normally.

Angiosperms perform double fertilization

·         The pollen grain consists of two cells.

·         The larger tube cell encloses the smaller generative cell.

·         During transport through the pollen tube, the generative cell undergoes one mitotic division and cytokinesis to produce two sperm cells.

·         Both sperm cells enter the embryo sac.

·         The sperm cells enter the cytoplasm of a synergid.

·         The synergid breaks down, and one sperm nucleus unites with the two polar nuclei, forming the 3n endosperm generation.

Embryos develop within seeds

·         After fertilization, the embryo, endosperm, integuments, and carpel develop.

·         The integuments become the seed coat.

·         The carpel becomes the wall of the fruit.

·         The zygote divides and the daughter cells have different fates.

·         The asymmetrical division of cytoplasm causes one end to produce the embryo and the other end the suspensor.

·         The longitudinal axis of the new plant is established. The embryo forms a primary meristem and then the first organs.

·         In eudicots, the globular embryo takes on a characteristic heart-shaped form.

·         Further elongation gives rise to the torpedo stage.

·         Large amounts of starch, lipid, and protein accumulate in the endosperm.

·         Later, the seed loses a lot of water.

Some fruits assist in seed dispersal

·         Fruit develops after fertilization.

·         A fruit consists of a mature ovary and its seeds.

·         Some fruits are dry or inedible.

·         Some fruits help disperse seeds over distances. Winged fruit can be blown by the wind.

·         Coconuts have spread from island to island by floating in the ocean.

·         Some seeds hitch rides on animals.

The Transition to the Flowering State

Apical meristems can become inflorescence meristems

·         The first sign of the flowering state may be changes in the apical meristems.

·         Flowers appear singly or in an orderly cluster that is called an inflorescence.

·         If a vegetative meristem becomes an inflorescence meristem, bracts and other new meristems may be produced.

·         The new meristems may be either inflorescence meristems or floral meristems.

Photoperiodic Control of Flowering

·         Life cycles of flowering plants fall into three categories: annual, biennial, and perennial.

·         Annuals complete their life cycle in less than a year.

·         Biennials live almost two years.

·         Perennials live for a few to many years.

·         Some plants require photoperiodic signals (a certain day length or night length) to flower.

·         'Maryland Mammoth' tobacco plants are mutant for flowering times.

·         They require at least 10 hours of darkness to flower.

·         Their critical day length is 14 hours.

There are short-day, long-day, and day-neutral plants

·         Poinsettias, chrysanthemums, and “Maryland Mammoth” tobacco plants are short-day plants, flowering only when day length is shorter than a critical maximum.

·         Spinach and clover are long-day plants.

·         Some plants, short–long-day plants, are more complex, requiring first short, then long days.

·         White clover is an example.

·         Day-neutral plants are most common; flowering is independent of daylight length.

The length of the night determines whether a plant will flower

·         Plants actually measure the length of darkness, not light.

·         Karl Hamner and James Bonner ran experiments where either the period of light or dark was held constant and the counterpart was varied.

·         They discovered it was the length of darkness that determined timing of flowering.

·         For cockleburs, just a single dark night is enough to trigger flowering some days later.

·         Plants measure continuous lengths of darkness. Interruptions during the dark period (not the light period) reset the dark time period.

·         Phytochromes are involved in the photoperiodic timing mechanism.

·         The Pfr form during the day is converted to the Pr form during the night.

·         This system is not adequate to explain the whole effect, though. Another yet-to-be-discovered system must exist.

Circadian rhythms are maintained by a biological clock

·         Some sort of biological clock resides in all eukaryotes.

·         The major outward manifestations are called circadian rhythms.

·         The cycle is measured by its length or period. Amplitude is the magnitude of the change over the course of a cycle.

·         The period is insensitive to temperature, but amplitude might be affected by it.

·         Circadian rhythms are highly persistent and continue even in an environment in which there is no alternation of light and dark.

·         Circadian rhythms can be entrained, within limits, by exposing the organism to light–dark regimes that deviate from 24 hours.

·         A brief exposure to light can shift the rhythm.

·         Many plants have observable cyclical changes.

·         Flowers of many plants close at night and open during the day.

·         They continue to follow this pattern even if kept in constant light or dark.

·         The photoperiodic behavior of plants is based on an interaction of night length and the biological clock.

Is there a flowering hormone?

·         Does the much-sought-after flowering hormone exist?

·         Experiments provide evidence that such a signal does exist.

·         The name florigen has been given to this flowering hormone, even though it has not been isolated and characterized.

·         Evidence suggests that the florigen of short-day plants is identical to that of long-day plants.

·         In one experiment, short- and long-day period plants were grafted together, and both flowered if the photoperiod was inductive to one of the partners.

Vernalization and Flowering

·         Spring wheat is an annual, while winter wheat is a biennial.

·         If winter wheat is not exposed to cold after its first year, it fails to flower normally the next year.

·         Experiments in Russia demonstrated that if winter wheat is moistened and chilled, it could be used as if it were spring wheat.

·         This process of inducing flowering by low temperatures is call vernalization.

·         Inducing flowering might require up to 50 days of low temperature.

Asexual Reproduction

·         Asexual reproduction is used by some plants.

·         Self-fertilization of a plant can produce both kinds of homozygotes, plus the heterozygote, among its progeny, but no new alleles are introduced.

·         Asexual reproduction goes farther: It involves no recombination, and homozygosity is not increased.

There are many forms of asexual reproduction

·         In vegetative reproduction, often the stem becomes modified.

·         Grass and strawberries produce stolons, horizontal stems that form runners with roots on their end.

·         Rhizomes are horizontal underground stems that can give rise to new shoots.

·         Bamboo reproduces this way.

·         Lilies and onions form bulbs.

·         Crocuses, gladioli, and many other plants produce corms. These underground stems are disclike and consist primarily of stem tissue.

·         Leaves may also be the source of new plantlets.

·         An example is the succulent plants of the genus Kalanchoe.

·         Plants that reproduce asexually often live on unstable earth, such as sand dunes.

·         Dandelions, citrus trees, and some other plants reproduce by apomixis, the asexual production of seeds.

·         Apomixis produces seeds within the female gametophyte without the mingling and segregation of chromosomes.

Asexual reproduction is important in agriculture

·         Cultivating plants by cutting stems, inserting them in soil, and waiting for them to form roots has been practiced for centuries.

·         Grafting—attaching a bud or a piece of stem from one plant to the root-bearing stem of another plant—is a common practice for fruit trees.

·         The root-bearing "host" is called the stock; the part grafted on is called the scion.

·         Most fruit for market in the United States is produced on trees grown from grafts.

·         Plant lice, Phylloxera sp., destroyed much of the grapevines in France.

·         Phylloxera-resistant roots from California were used on the French vines to save them by providing resistance.

·         Universities produce valuable plant materials via tissue culture.

·         Tissue cultures are used commercially to produce orchids, rhododendrons, and many crops.

·         Recombinant DNA techniques applied to tissue cultures can provide plants with capabilities they previously lacked, such as resistance to pests or increased nutritive value.