Water movement through a plant

 

Xylem vessels are long, narrow, hollow tubes containing no living material and impregnated with bands or spirals of lignin. Joined end to end, they provide a continuous pathway from the roots through the stem and to the leaves. As well as being waterproof, the lignin provides strength to stop the vessels from collapsing. The narrow diameters of the xylem vessels increase the capillarity forces, however once again, this only makes a small contribution to water's upward movement.

  *Xylem vessels in stem LS

Looking at leaf structure provides the clues to what is the main force involved in maintaining a continuous flow of water from root to leaf through the xylem.

In broad leaved plants, the leaves are large, thin, flat structures. Large, in order to trap lots of light energy, and thin so that the carbon dioxide can diffuse into the leaf from the surrounding air.

Examination of a leaf reveals small pores, or stomata, in the leaf's surfaces. Each stoma is controlled by 2 guard cells which "open" and "close" the pore. If the plant is to obtain sufficient carbon dioxide for photosynthesis it is necessary that the stomata are open, however, in being open, water can be lost through these same stomata by evaporation. It is this evaporation, known as transpiration, which is the driving force for pulling water through a plant. In most broad leaved plants, a greater number of stomata are found on the cooler, lower surface. This ensures that sufficient carbon dioxide can enter while at the same time cutting down the amount of water lost by transpiration.


TS Privet leaf


 



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