Growth Confined to Zones
A plant does not grow all over. Division is confined to meristems, regions of permanently undifferentiated cells, and everything else elongates or differentiates but does not divide. This is why a nail driven into a trunk stays at the same height for a century.
Meristems divide by what they add:
| Meristem | Location | Adds |
|---|---|---|
| Apical | Shoot and root tips | Length (primary growth) |
| Lateral (cambium) | Cylinders within stem and root | Girth (secondary growth) |
| Intercalary | At the base of internodes | Length, in grasses |
Apical meristems drive primary growth. Behind the tip sit three zones in sequence — division, elongation and differentiation — so a root tip is a spatial record of a temporal process: cells born at the tip are pushed backwards through each stage as new ones form behind them.
Lateral meristems produce secondary growth. The vascular cambium adds xylem inward and phloem outward, and in a seasonal climate the difference between fast spring xylem and dense summer xylem is what makes a visible growth ring. The cork cambium produces the outer protective bark.
Intercalary meristems, at the bases of internodes, are why grasses regrow after being cut or grazed, the growing zone is below the cut, not at the tip. Lawns and grazing systems depend entirely on this.
The epidermis is the covering tissue of the primary plant body: usually a single layer, secreting a waxy cuticle that is the main barrier to water loss. But a perfect barrier would also block CO₂, so the epidermis is perforated by stomata, each flanked by two guard cells that open and close it.
Guard cells work hydraulically. They take up potassium ions, water follows by osmosis, and their unevenly thickened walls make them bow apart rather than simply swell, opening the pore. Losing that water closes it. Most plants open by day and close at night; desert CAM plants reverse this entirely, opening at night to lose less water.
Common pitfall: thinking a plant grows by its existing cells all enlarging. Growth in length comes from meristems adding new cells at the tips, so a branch never rises higher up the trunk, its height was fixed when the meristem that made it passed that point.