Plant Structure and Physiology

Main notes

Plants feed nearly every terrestrial food web, yet a plant body is mostly plumbing, scaffolding, and signals. This chapter covers the tissue systems, the anatomy of roots and shoots, the transport streams of xylem and phloem, nutrition, hormones, and the movements and seasonal responses plants make. It follows the chapter on plant diversity and sets up the ecology chapters, where water use and nutrients shape ecosystems.

Tissue Systems

The plant body is built from three tissue systems: the dermal, ground, and vascular systems. The dermal system is the outer covering: the epidermis, a layer of cells coated with a waxy cuticle, dotted with stomata flanked by guard cells that open and close each pore to balance carbon dioxide uptake against water loss. Trichomes reduce evaporation, and root hairs absorb water; in woody stems and roots, periderm (bark) replaces the epidermis. The ground tissue system fills the interior. Parenchyma cells are thin walled, alive, and do photosynthesis and starch storage; collenchyma cells have unevenly thickened walls for flexible support; sclerenchyma cells die at maturity after building thick, lignified walls that give rigid support. The vascular tissue system is the plumbing: xylem moves water and minerals, and phloem moves sugars. All three systems come from meristems, with apical meristems at the root and shoot tips producing primary growth.

Common Mistake: Treating every plant cell as alive. Sclerenchyma cells are dead at maturity and serve purely structural roles, and mature xylem water-conducting cells are also dead tubes. Ground tissue is not all photosynthetic; collenchyma and sclerenchyma mostly provide support.

ELI-10

Think of a plant as a raincoat with padding and pipes built in. The outer shell keeps the water out, the soft padding stores lunch and makes food, and the pipes move drinks around. Each part has a job, and the plant works best when all three work together.

Root and Shoot Anatomy

Roots anchor the plant and absorb water and minerals. Behind the root cap, the apical meristem produces cells, and the zone of elongation pushes the root through the soil. In the zone of maturation, cells differentiate and root hairs grow out of the epidermis, multiplying the absorptive surface. Water and ions cross the cortex to the endodermis, whose Casparian strip, a waterproof band, forces everything entering the vascular cylinder to cross living membranes. The pericycle can produce branch roots. Eudicot roots have a central star of xylem with phloem between its arms; monocot roots have a ring of bundles around a pith.

Shoots are stems and leaves. A stem has nodes, where leaves attach, and internodes between them; terminal buds and axillary buds hold meristems, and apical dominance suppresses the axillary buds until the tip is removed. Eudicot stems arrange vascular bundles in a ring; monocot stems scatter them. The leaf is a blade on a petiole: the palisade mesophyll does most photosynthesis, the spongy mesophyll below has air spaces that open at stomata, and veins lace through both layers.

ELI-10

A sponge soaks up more juice than a solid cube of bread because its surface is covered in tiny holes. Root hairs work the same way, making the root's skin vastly bigger. A bigger surface means more water and minerals can slip in.

ELI-10

The palisade and spongy layers of a leaf are like solar panels above an open parking garage. The panels are packed tightly on top to catch the most sunlight, while the open space below lets gases move around. Stomata are the garage doors, opening to let carbon dioxide in.

Xylem and Transpiration

Water potential sets the direction of water movement: it equals the solute potential plus the pressure potential. Pure water has a water potential of zero, and adding solutes makes it negative. Water moves from higher (less negative) to lower (more negative) water potential, so it enters roots only while the soil is less negative than the leaf end of the column.

Transpiration is the evaporation of water from leaves, mostly through stomata; about 90 to 99 percent of absorbed water is lost this way, and only a small fraction stays for growth and photosynthesis. That loss is not waste, because it drives transport. In the cohesion-tension mechanism, evaporation creates tension, and cohesion of water molecules through hydrogen bonds passes that pull down the column to the roots, where adhesion to xylem walls helps. The stomatal pore is the valve: guard cells that take up potassium swell and open the pore, and losing potassium, or receiving abscisic acid, closes it.

1. Sunlight evaporates water from mesophyll cell walls at the stomata.
2. Evaporation lowers the water potential of the air spaces in the leaf.
3. Water exits leaf cells, pulling water out of the xylem stream.
4. Cohesion through hydrogen bonds passes the pull down the column.
5. The tension reaches the roots, and root hairs take up replacement water.
6. Adhesion keeps the water film stuck to the xylem walls along the way.

Common Mistake: Reversing the water potential gradient. Water moves from higher (less negative) to lower (more negative) water potential, even when that means moving toward a higher solute concentration. Root pressure alone cannot push water to a canopy; transpiration does the pulling.

ELI-10

Drinking a very long smoothie with a straw, one sip at the top moves the whole column because each drop drags the next drop along. In a tree, the sun does the sipping at the top of a very long straw. Each water molecule pulls its neighbor, like a chain of people holding hands being tugged from the front.

Phloem and Translocation

Translocation is the transport of sugars in phloem from sources to sinks. A source produces or releases sugar, such as a leaf in sunlight or a storage organ in spring; a sink uses or stores it, such as growing roots, developing fruits, and young leaves. Phloem is built from sieve-tube elements joined end to end through perforated sieve plates, each tube partnered with companion cells, which supply the nucleus and organelles the sieve tubes lack and load sucrose through plasmodesmata. Sugar travels as sucrose, which is less reactive than glucose.

By the pressure-flow hypothesis, loading sucrose at the source lowers water potential, water enters from the xylem by osmosis, and pressure rises; unloading at the sink releases sucrose, water follows, and pressure falls. The resulting gradient pushes sap from source to sink, and since different tubes run opposite directions, sap can move up, down, or sideways.

1. At the source, companion cells actively pump sucrose into sieve tubes.
2. The concentrated sap draws water in from the adjacent xylem.
3. Water entry raises the pressure at the source end of the tube.
4. At the sink, sucrose is unloaded and used or stored.
5. Water exits with the sucrose, lowering pressure at the sink.
6. The pressure difference drives bulk flow from source to sink.

Common Mistake: Treating phloem as a second transpiration stream. Sugar moves by pressure flow driven by active loading, not by evaporation, and it always travels from source to sink. A sink is defined by use, not position: a storage root can be a sink in summer and a source in spring.

ELI-10

Imagine two water balloons connected by a wide straw, with candy being stuffed into one balloon. The candy balloon swells and squeezes its sugary water through the straw into the emptier balloon. The plant stuffs sugar in at every leaf, and the extra pressure pushes the sweet sap to the parts that need it.

Nutrition

Plants need about 17 essential elements. Macronutrients are required in large amounts: carbon, hydrogen, and oxygen come from carbon dioxide and water, while nitrogen, phosphorus, and potassium (the N-P-K on fertilizer bags), plus calcium, magnesium, and sulfur, come from the soil. Nitrogen builds amino acids, proteins, and nucleic acids; phosphorus builds ATP and nucleic acids; potassium regulates stomata and enzymes; and magnesium sits at the center of every chlorophyll molecule. Micronutrients, including iron, manganese, zinc, copper, boron, molybdenum, chlorine, and nickel, are needed in trace amounts, mostly as enzyme cofactors. Deficiencies cause chlorosis (yellowing); nitrogen and magnesium move out of old leaves, while iron does not.

Root hairs absorb mineral ions by active transport, and ions must dissolve first, so dry soil starves plants even when nutrients are present; soil pH controls availability. Nitrogen-fixing bacteria such as Rhizobium convert atmospheric nitrogen into ammonia in legume nodules, and mycorrhizae are fungi whose hyphae extend the root's reach, especially for phosphorus.

Common Mistake: Believing a plant builds most of its body from soil. The carbon in every cellulose molecule comes from carbon dioxide in the air, and oxygen and hydrogen come from water; soil nutrients contribute only a small fraction of plant dry mass.

ELI-10

A plant is like an athlete who needs protein to build muscle, salts to keep nerves firing, and tiny amounts of vitamins for the small jobs. Miss one item and a specific symptom shows up, like a leaf turning yellow. Fertilizer is just a way to serve the plant its full menu.

Hormones

Five classes of hormones coordinate plant life. Auxins promote cell elongation, maintain apical dominance, and drive phototropism bending; synthetic auxins act as herbicides. Cytokinins promote cell division and shoot growth, and the auxin-to-cytokinin ratio decides whether tissue becomes root or shoot. Gibberellins stimulate stem elongation, seed germination, and fruit growth. Abscisic acid (ABA) is the stress hormone: it closes stomata during drought, promotes seed dormancy, and inhibits growth. Ethylene is a gaseous hormone that triggers fruit ripening, leaf abscission (leaf drop), and senescence, which is why one ripening fruit can ripen its neighbors.

Common Mistake: Assuming every plant hormone promotes growth. Abscisic acid is mostly inhibitory, closing stomata and forcing dormancy, and ethylene triggers ripening and aging.

ELI-10

Hormones are like text messages sent across the plant. A drought-stressed root texts the leaves to close their doors and save water. A ripening fruit texts its neighbors, which is why one bad apple really can spoil the bunch.

Tropisms and Photoperiodism

Tropisms are directional growth responses to environmental cues. In phototropism, a shoot bends toward light because auxin moves to the shaded side and elongates those cells. In gravitropism, dense statoliths settle with gravity and redistribute auxin; because root cells and shoot cells respond to high auxin oppositely, shoots grow away from gravity while roots grow toward it. Thigmotropism is growth in response to touch, as when tendrils coil around a support.

Photoperiodism is the response to day length that times flowering. Short-day plants flower when nights exceed a critical length, so they are really long-night plants; long-day plants flower when nights are shorter than critical; day-neutral plants flower regardless. The clock is the pigment phytochrome, which converts between two forms: red light turns Pr into the active Pfr, while far-red light, or a long night, turns Pfr back into Pr. The Pfr level at dawn reports night length, which is why a flash of red light in the middle of the night can cancel a short-day plant's flowering signal.

1. Red light converts phytochrome from Pr into the active Pfr form.
2. Far-red light converts Pfr back into the inactive Pr form.
3. During long nights, Pfr slowly reverts to Pr on its own.
4. The Pfr level remaining at dawn tells the plant how long the night was.

Common Mistake: Reading "short-day plant" as a preference for short days. What matters is the uninterrupted night: a short-day plant flowers on long nights, and a flash of red light in the night can prevent flowering even in short days.

ELI-10

A seedling in a dim room bends toward the window like a crowd leaning toward a stage. The side away from the light grows a little longer, so the stem tips toward the light. Roots follow gravity instead, growing downward even when light shines from the side.

ELI-10

Think of phytochrome as a switch inside every leaf. Red light flicks the switch on, and darkness slowly flicks it off. The plant remembers how long the switch stayed off, and that tells it the season.

High-Yield:

• Water moves from higher to lower water potential, and transpiration pull, not root pressure, moves it up the tree.
• Xylem carries water and minerals; phloem carries sugars by pressure flow from source to sink.
• Remember the five hormone classes by effect: auxins, cytokinins, gibberellins, abscisic acid, and ethylene.
• Short-day plants flower on long nights, and phytochrome converts between Pr and Pfr with red and far-red light.
• About 90 to 99 percent of absorbed water is lost to transpiration, so stomatal control matters enormously.

Quick Review

• The three tissue systems are dermal, ground, and vascular; sclerenchyma and mature xylem cells are dead.
• Roots absorb through root hairs and the endodermis, where the Casparian strip forces water across living membranes.
• Water potential equals solute potential plus pressure potential, and water moves toward the lower (more negative) value.
• Cohesion through hydrogen bonds lets transpiration at the leaf pull the entire water column.
• Phloem uses pressure flow: active loading of sucrose at the source pushes sap to the sink.
• Macronutrients include nitrogen, phosphorus, and potassium; micronutrients act mostly as cofactors.
• The five hormone classes are auxins, cytokinins, gibberellins, abscisic acid, and ethylene.
• Phytochrome switches between Pr and Pfr, so short-day and long-day plants flower on the right nights.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.