Plant Tissues and Growth

Meristematic tissue = regions of undifferentiated cells that retain the ability to divide by mitosis throughout the plant's life. They are analogous to stem cells in animals: their daughter cells can differentiate into any plant cell type, while the meristem itself is self-renewing.

Apical meristems sit at the tips of roots and shoots and drive PRIMARY growth — the lengthening of roots and shoots. New cells produced here elongate and differentiate, extending the plant upward and downward.

Lateral meristems (the vascular cambium and cork cambium) form cylinders along the stem and root and drive SECONDARY growth — the widening/thickening of stems and roots (e.g. wood and bark in trees). SL only needs the apical-vs-lateral distinction.

Plant organs.

• Roots — anchor the plant and absorb water and mineral ions; root hairs increase surface area.
• Stems — support leaves, transport materials, and may store food.
• Leaves — the main sites of photosynthesis and gas exchange.

Tissue systems. Every plant organ is built from three tissue systems:

• Dermal tissue — the outer "skin" (epidermis), often with a waxy cuticle; protects and limits water loss.
• Ground tissue — the bulk of the plant; carries out photosynthesis (e.g. mesophyll), storage and support.
• Vascular tissue — the transport system: xylem (water + minerals, upward) and phloem (sugars, in both directions).

Indeterminate growth. Unlike most animals, plants grow indeterminately — they keep adding organs and tissue from meristems throughout life, with no fixed final size.

A tropism is a directional growth response to an external stimulus. The direction is set relative to where the stimulus comes from:

• Phototropism — growth in response to light (shoots grow toward light = positive phototropism).
• Gravitropism (geotropism) — growth in response to gravity (roots grow downward = positive; shoots grow upward = negative).

Auxin (indole-3-acetic acid, IAA) is the plant hormone that controls these responses by altering the rate of cell elongation.

Phototropism mechanism (shoot bending toward light):

1. Light strikes one side of the shoot tip.
2. Auxin is actively redistributed to the SHADED side of the shoot.
3. The higher auxin concentration there promotes cell elongation on the shaded side.
4. The shaded cells lengthen more than the lit cells, so the shoot bends toward the light.

PIN proteins (auxin efflux carriers). Auxin gradients are not random — they are set up by PIN proteins, membrane transporters that pump auxin OUT of cells. Because PIN proteins are positioned asymmetrically in the cell membranes, they channel auxin directionally (e.g. toward the shaded side, or down toward the root tip), establishing the concentration gradients that drive the growth response.

Same hormone, opposite effects. Auxin's effect depends on the tissue and concentration:

• In SHOOTS, the concentrations reached PROMOTE elongation.
• In ROOTS, the same/higher concentrations INHIBIT elongation.

This neatly explains gravitropism: when a root is laid sideways, auxin accumulates on the lower side; in the root that inhibits elongation there, so the upper side grows faster and the root curves downward. In a shoot, lower-side auxin promotes elongation, so the shoot curves upward.

Cell totipotency and micropropagation. Many plant cells are totipotent — a single differentiated cell can dedifferentiate and regenerate a whole new plant. Micropropagation (tissue culture) exploits this: small pieces of tissue are grown on a sterile nutrient medium with hormones to produce many genetically identical plantlets (clones), useful for rapid multiplication, disease-free stock and propagating valuable or hard-to-grow varieties.