# Plant Ground and Vascular Tissues

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

## Filling, Stiffening, Plumbing

Beyond the epidermis, the plant body is built from three ground tissues and two vascular ones.

**Parenchyma** is the general-purpose tissue: living cells with thin primary walls, doing photosynthesis, storage and repair. It retains the ability to divide, which is why a cutting can regenerate a whole plant.

**Collenchyma** has unevenly thickened primary walls and is alive at maturity. It provides flexible support to growing organs, the strings in a celery stalk are collenchyma, and being alive it can extend as the organ grows.

**Sclerenchyma** has thick lignified secondary walls and is dead at maturity. It provides rigid support, in long **fibres** or in stubby **sclereids**: the grittiness of a pear is sclereids.

The two vascular tissues run the plumbing.

**Xylem** carries water and minerals upward in dead, hollow, lignified cells. **Tracheids** are narrow and taper, passing water through pits; **vessel elements** are wider and stack into continuous open tubes. Vessels move far more water, and the reason is dramatic. Flow through a tube follows Poiseuille's law:

$$Q \propto r^4$$

Doubling the radius multiplies flow by $2^4 = 16$. Tripling it multiplies flow by 81. A small change in vessel diameter transforms conducting capacity, which is why the evolution of wide vessels was so consequential, and why the trade-off against cavitation risk matters so much.

**Phloem** carries sugars in both directions, from source to sink, in living **sieve-tube elements**. These lose their nucleus at maturity and are kept alive by adjacent **companion cells**: an unusual arrangement in which one cell maintains another's metabolism.

In organography these tissues arrange differently by organ. Roots have a central vascular cylinder that resists pulling; stems have vascular bundles arranged for bending resistance; leaves spread them as veins through photosynthetic tissue.

> **Common pitfall:** assuming a wider xylem vessel is simply better. It moves far more water, but a wide column of water under tension is much more vulnerable to **cavitation**, an air bubble breaking the column and disabling the vessel permanently. Plants in dry or freezing climates often have narrower, safer vessels on purpose.

## Practice questions

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

### 1. Flow through a xylem vessel is proportional to the fourth power of its radius. If one vessel has three times the radius of another, how many times more water does it carry?

**Answer:** 81 (within ±1)

**Why:** $3^4 = 81$ times more water. A single wide vessel can outperform dozens of narrow ones, which is why angiosperms with wide vessels can support far higher transpiration rates than conifers with tracheids alone.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/flow-through-a-xylem-vessel-is-proportional-to-the-fourth-power-of

### 2. Why do plants in cold or dry climates often have narrower xylem vessels despite the enormous flow advantage of wide ones?

A. Narrow vessels prevent secondary wall collapse under high tension without lignin
B. Wide vessels are far more vulnerable to cavitation breaking the water column
C. Narrow vessels reduce total carbon investment per unit of water transported
D. Narrow vessels generate higher capillary lift to draw water up taller stems

**Answer:** B. Wide vessels are far more vulnerable to cavitation breaking the water column

**Why:** A wide column under tension is much more likely to cavitate, and a cavitated vessel is permanently lost. Freezing and drought both promote bubble formation, so plants in those climates trade conducting capacity for safety.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/why-do-plants-in-cold-or-dry-climates-often-have-narrower-xylem

### 3. Collenchyma provides mechanical support while remaining alive at maturity, whereas sclerenchyma dies once its secondary wall forms. What functional consequence follows from keeping a supporting cell alive?

A. It can develop thick lignified walls without arresting growth
B. It can generate metabolic energy to drive active sap movement
C. It can stretch and expand as the surrounding organ elongates
D. It can divide repeatedly to regenerate damaged vascular bundles

**Answer:** C. It can stretch and expand as the surrounding organ elongates

**Why:** Lignified sclerenchyma cannot expand once set, making it unsuitable for growing tips. Retaining a living protoplast allows collenchyma to remodel its primary wall during elongation. Active transport in plumbing belongs to phloem, and post-injury cell division is the domain of unspecialised parenchyma.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/collenchyma-provides-mechanical-support-while-remaining-alive-at

### 4. Xylem carries water in cells that are dead at maturity.

**Answer:** True

**Why:** True, xylem conducting cells die and empty, leaving a hollow lignified tube. The contents would obstruct flow, so death is a functional requirement rather than a limitation. Phloem, by contrast, must stay alive to move sugars actively.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/xylem-carries-water-in-cells-that-are-dead-at-maturity

### 5. Sort each plant tissue by its primary function.

**Answer:**

- Support: Collenchyma, Sclerenchyma fibres
- Transport: Xylem, Phloem
- General-purpose: Parenchyma

**Why:** Collenchyma and sclerenchyma support, xylem and phloem transport, and parenchyma does photosynthesis, storage and repair. Parenchyma’s retained ability to divide is why cuttings can regenerate whole plants.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/sort-each-plant-tissue-by-its-primary-function

### 6. Arrange these steps in the order a xylem vessel element becomes functional.

**Answer:**

1. The cell is produced by the vascular cambium
2. It elongates and deposits a lignified secondary wall
3. The end walls are broken down, connecting it to its neighbours
4. The cell dies and its contents are cleared
5. Water flows through the continuous hollow tube

**Why:** The cell builds its rigid wall first, then opens its end walls and dies, leaving a hollow lignified pipe. Building the wall before dying is essential, a cell that died first could not construct the tube it becomes.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/arrange-these-steps-in-the-order-a-xylem-vessel-element-becomes

### 7. Which statements about phloem are correct?

A. Sieve-tube elements lose their nucleus at maturity
B. It transports sugars from source to sink
C. Companion cells maintain the sieve-tube elements
D. Transport occurs only upward, as in xylem

**Answer:** A. Sieve-tube elements lose their nucleus at maturity; B. It transports sugars from source to sink; C. Companion cells maintain the sieve-tube elements

**Why:** Phloem moves sugars from source to sink, in whichever direction that requires, down to roots, up to fruits, or both at once in different tubes. Only xylem is strictly unidirectional.

Page: https://tryals.app/practice/biology-i/plant-ground-and-vascular-tissues/which-statements-about-phloem-are-correct