Plant Form and Physiology

1. Tissues and organs

Plants have three tissue systems. The dermal tissue is the outer covering — the epidermis with its waxy cuticle and stomata in leaves and stems, and the periderm (bark) in woody plants; root hairs extend the epidermis for absorption. The vascular tissue is the transport system: xylem moves water and dissolved minerals upward, while phloem moves sugars and other organic products throughout the plant. The ground tissue fills the space between — parenchyma (thin-walled, for photosynthesis and storage), collenchyma (flexible support), and sclerenchyma (rigid, often dead, support). These tissues build three organs: roots (anchor and absorb), stems (support and conduct), and leaves (main site of photosynthesis).

2. Meristems and growth

Plants grow from meristems, regions of undifferentiated cells that divide continuously. Apical meristems at the tips of roots and shoots produce primary growth, lengthening the plant. In woody plants, lateral meristems — the vascular cambium (which adds xylem and phloem) and the cork cambium (which produces bark) — produce secondary growth, thickening the stem and root. This is why a tree gains both height and girth, and why "wood" is simply accumulated secondary xylem.

3. Water transport: the cohesion–tension mechanism

Water moves upward through xylem by transpiration pull. Water evaporates from leaf cells and exits through stomata (transpiration); this creates tension that pulls the water column upward. Cohesion (water molecules hydrogen-bonding to each other) and adhesion (water molecules clinging to xylem walls) keep the column from breaking, so the whole chain is drawn up from the roots as evaporation continues. Stomata, opened and closed by pairs of guard cells, balance the need to take in carbon dioxide against the risk of losing too much water.

4. Sugar transport: the pressure–flow mechanism

Sugars move through phloem by pressure–flow (bulk flow). At a source (usually a mature leaf making sugar), sugar is actively loaded into sieve-tube elements, drawing water in by osmosis and raising pressure; at a sink (roots, fruits, growing tips), sugar is unloaded, lowering pressure. The pressure difference pushes the sugar-rich sap from source to sink through the living sieve-tube elements and their companion cells.

5. Nutrition

Plants obtain carbon and oxygen from the air but take most mineral nutrients from the soil through their roots. Essential elements include macronutrients such as nitrogen, phosphorus, and potassium and micronutrients such as iron and zinc. Many plants form mycorrhizae, mutualistic associations between roots and fungi that greatly increase the surface area for absorbing water and minerals, and legumes host nitrogen-fixing bacteria in root nodules.

How it works

How water climbs a tall tree (cohesion–tension):

1. Sunlight drives photosynthesis, and stomata open to admit carbon dioxide.
2. Water evaporates from moist cell walls inside the leaf and diffuses out through the stomata (transpiration).
3. This evaporation pulls water out of the leaf's xylem, creating tension (negative pressure).
4. Cohesion transmits that tension down the continuous water column, and adhesion keeps the column against the xylem walls.
5. The tension reaches the roots, drawing in more water and dissolved minerals from the soil.
6. The result is a steady upward flow with no pump — powered by evaporation at the top.

Common confusions

Memory aids

Use the letter clues: Xylem carries water (both start with a "wa"-like sound — think "xylem = water, upward"), and Phloem carries Phood (sugars). For the pull of water, remember "C.A.T." — Cohesion, Adhesion, Transpiration — the three forces that raise water to the treetops.

Quick review

Topic Recap

• Three tissue systems (dermal, vascular, ground) build three organs (roots, stems, leaves).
• Meristems drive growth: apical for primary growth, lateral for secondary growth.
• Xylem transports water upward by cohesion–tension, powered by transpiration.
• Phloem distributes sugars by pressure–flow from source to sink.
• Stomata regulate gas exchange and water loss; roots, mycorrhizae, and nitrogen-fixing bacteria supply nutrition.

Knowledge Check

1. Which tissue transports water, and by what mechanism?
2. What is the difference between primary and secondary growth?
3. What force actually pulls water up a tree?
4. In pressure–flow, what drives the movement of sugars from source to sink?
5. What role do guard cells play in balancing gas exchange and water loss?

Answers and Rationales

1. Answer: Xylem, by cohesion–tension (transpiration pull). Why: Evaporation from leaves creates tension that draws the cohesive water column upward.
2. Answer: Primary growth lengthens the plant via apical meristems; secondary growth thickens it via lateral meristems. Why: They come from different meristems and add different dimensions.
3. Answer: Transpiration — evaporation of water from the leaves. Why: It creates the tension transmitted down the water column; there is no pump.
4. Answer: A pressure difference created by loading sugar at the source and unloading it at the sink. Why: Water follows the sugar by osmosis, building pressure that pushes sap toward the sink.
5. Answer: Guard cells open and close the stomata. Why: Open stomata admit CO₂ for photosynthesis but allow water loss, so their control balances the two.