# Plant Meristems and Covering Tissues

Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/plant-meristems-and-covering-tissues

## 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.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Why does a nail driven into a tree trunk stay at the same height for decades?

A. The nail destroys the local cambium, halting upward elongation there
B. Growth in length happens only at the tips, not throughout the trunk
C. Secondary xylem pushes the nail outward at the rate the trunk elongates
D. Woody stems grow exclusively in diameter and never extend in length at all

**Answer:** B. Growth in length happens only at the tips, not throughout the trunk

**Why:** Length is added at apical meristems only. Everything already formed stays where it is, so the nail keeps its height while the trunk thickens around it, which is also why a branch never climbs the trunk.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/why-does-a-nail-driven-into-a-tree-trunk-stay-at-the-same-height-for

### 2. Sort each meristem by what it adds to the plant.

**Answer:**

- Adds length: Apical meristem at the shoot tip, Apical meristem at the root tip, Intercalary meristem at an internode base
- Adds girth: Vascular cambium, Cork cambium

**Why:** Apical and intercalary meristems produce primary growth in length; the two cambia produce secondary growth in girth. A plant that only ever had apical meristems would be tall and permanently thin.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/sort-each-meristem-by-what-it-adds-to-the-plant

### 3. A grass plant is mown, removing the top of every leaf. Set the fraction of its growing capacity that is lost, from 0 (none) to 1 (all of it).

**Answer:** 0 (within ±0.12)

**Why:** Essentially **none** is lost. Grasses grow from intercalary meristems at the base, below the mower blade, which is exactly why lawns tolerate mowing and grasslands tolerate grazing while most other plants do not.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/a-grass-plant-is-mown-removing-the-top-of-every-leaf-set-the

### 4. The waxy cuticle covering a leaf is perforated by stomata.

**Answer:** True

**Why:** True, a perfectly sealed leaf would not lose water but would also starve, since CO₂ could not enter. Stomata are the compromise: a controllable breach in the barrier, opened only when the exchange is worth the water.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/the-waxy-cuticle-covering-a-leaf-is-perforated-by-stomata

### 5. A root tip is described as 'a spatial record of a temporal process'. What does this distinction imply about how a plant organ develops over time?

A. Cells remain in place whilst undergoing their mature differentiation
B. Older cells sit further from the tip than newly divided neighbours
C. Cell division continues uniformly along the whole elongation region
D. Tissues migrate forwards through the tip to assume their final role

**Answer:** B. Older cells sit further from the tip than newly divided neighbours

**Why:** Differentiating cells are physically left behind as the apical meristem drives forward; they do not migrate or continue dividing throughout the organ. Recognising this spatial-temporal mapping explains why cell lineages remain fixed in position along the axis.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/a-root-tip-is-described-as-a-spatial-record-of-a-temporal-process

### 6. Which statements about secondary growth are correct?

A. Growth rings arise from seasonal differences in xylem
B. It is produced by lateral meristems
C. The vascular cambium adds xylem inward and phloem outward
D. It increases the length of the stem

**Answer:** A. Growth rings arise from seasonal differences in xylem; B. It is produced by lateral meristems; C. The vascular cambium adds xylem inward and phloem outward

**Why:** Secondary growth thickens the stem: the vascular cambium lays down xylem inward and phloem outward, and seasonal variation in that xylem produces visible rings. Length comes only from apical meristems.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/which-statements-about-secondary-growth-are-correct

### 7. A CAM desert plant is running its daily cycle. Arrange these events in the order they occur, starting at dusk.

**Answer:**

1. Stomata open at night, when the air is cool and humid
2. Carbon dioxide is taken up and stored as an organic acid
3. Stomata close at dawn, sealing the leaf against water loss
4. The stored acid releases carbon dioxide for photosynthesis in daylight

**Why:** CAM plants uncouple carbon uptake from photosynthesis: they gather CO₂ at night when evaporation is lowest, store it chemically, and use it by day with the stomata shut. It costs energy and buys an enormous saving in water.

Page: https://tryals.app/practice/biology-i/plant-meristems-and-covering-tissues/a-cam-desert-plant-is-running-its-daily-cycle-arrange-these-events