VASCULAR BUNDLES

VASCULAR BUNDLES

Plants have two systems for the transportation of substances - using two different types of transport tissue. Xylem transports water and solutes from the roots to the leaves, while phloem transports food from the leaves to the rest of the plant. Transpiration is the process by which water evaporates from the leaves, which results in more water being drawn up from the roots. Plants have adaptations to reduce excessive water loss.

Xylem and phloem

Plants have two transport systems to move food, water and minerals through their roots, stems and leaves. These systems use continuous tubes called xylem and phloem, and together they are known as vascular bundles.

Plant stem

Stem – the xylem and phloem are arranged in bundles near the edge of the stem to resist compression and bending forces.

Plant root

Root - xylem and phloem in the centre of the root to withstand stretching forces.

Xylem

Xylem vessels are involved in the movement of water through a plant - from its roots to its leaves via the stem.

During this process:

1. Water is absorbed from the soil through root hair cells.
2. Water moves by osmosis from root cell to root cell until it reaches the xylem.
3. It is transported through the xylem vessels up the stem to the leaves.
4. It evaporates from the leaves (transpiration).

The xylem tubes are made from dead xylem cells which have the cell walls removed at the end of the cells, forming tubes through which the water and dissolved mineral ions can flow. The rest of the xylem cell has a thick, reinforced cell wall which provides strength.

Phloem

Phloem vessels are involved in translocation. Dissolved sugars, produced during photosynthesis, and other soluble food molecules are moved from the leaves to growing tissues (eg the tips of the roots and shoots) and storage tissues (eg in the roots).

In contrast to xylem, phloem consists of columns of living cells. The cell walls of these cells do not completely break down, but instead form small holes at the ends of the cell. The ends of the cell are referred to as sieve plates. The connection of phloem cells effectively forms a tube which allows dissolved sugars to be transported.

Transpiration

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. 

leaf

More water is drawn out of the xylem cells inside the leaf to replace what has been lost. Water molecules have a tendency to stick together – so as water leaves the xylem to enter the leaf, more water is pulled up behind it. This produces a continuous flow of water and dissolved minerals moving up the xylem tube from the roots, up the stem, and into the leaves. This is known as the transpiration stream.

Movement of water through the roots

The movement of water up the xylem means more water must be drawn in through the roots from the soil. To do this, water passes from root cell to root cell by osmosis.

The pathway of water across a root

As water moves into the root hair cell down the concentration gradient, the solution inside the root hair cell becomes more dilute. This means that there is now a concentration gradient between the root hair cell and adjacent root cells, so water moves from the root hair cell and into the adjacent cells by osmosis.
This pattern continues until the water reaches the xylem vessel within the root - where it enters the xylem to replace the water which has been drawn up the stem.

Factors that affect transpiration rate

Light

Transpiration increases in bright light. The stomata open wider to allow more carbon dioxide into the leaf for photosynthesis. More water is therefore able to evaporate. 

Temperature

Transpiration is faster in higher temperatures. Evaporation and diffusion are faster at higher temperatures.

Wind

Transpiration is faster in windy conditions. Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf.

Humidity

Transpiration is slower in humid conditions. Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.

Factors that speed up transpiration will also increase the rate of water uptake from the soil. If the loss of water is faster than the rate at which it is being replaced by the roots, then plants can slow down the transpiration rate by closing some of their stomata. This is regulated by guard cells, which lie on either side of a stoma.

Plants can slow down the transpiration rate by closing some of their stomata

If the guard cells are turgid, then they curve forming ‘sausage-shaped’ structures with a hole between them. This is the stoma.
However, if the guard cells are flaccid due to water loss, they shrivel up and come closer together, closing the stoma. This is turn reduces the water loss due to transpiration, and can prevent the plant from wilting.

By

Vinesh S