the root hairs. This is passed into the xylem vessels which form a continuous
system of tubes through root and stem into the leaves. Here the water
evaporates and releases into the atmosphere. The evaporation creates the
main pull of water above root pressure which gives a variable and minor
push from below. This results in a continuous column of moving water,
the ‘transpiration stream’.
Is there any relation between transpiration and rain fall?
The amount of water passing through a plant is often considerable.
For example, an oak tree can transpire as much as 900 liters of water per
day. It follows therefore that areas of forest significantly affect the degree
of saturation of the air above them, so that when air currents bring air
which is already nearly saturated to a forest area, it becomes fully saturated
and comes down as rain; this is why forest areas often have a higher rainfall
than areas nearby.
Do you know?
Transport of mineral salts
You know that mineral salts are necessary for plant nutrition (micro
and macro nutrients) and that they are obtained from the soil solution
through the root hairs. The salts are in the form of electrically charged
ions. Sodium chloride (NaCl) is in the form of Na+
and Cl-
, and Magnesium
Sulphate (MgSO4
) occurs as Mg2+ and SO42-. But, they are not absorbed
into the root hairs by the simple process of diffusion, but it involves the
use of energy by the cytoplasm which will be discussed in later classes.
Once ions are absorbed, the ions travel along with water in the xylem vessels
and pass to the growing points of the plants where they are used for growth
purpose. They may also pass laterally from xylem to phloem. Thus, mineral
salts are one of the natural factors in plant growth phenomena.
Transport of materials in plants
Food such as sugar is synthesised in the green parts of plants, mainly
the leaves, but this food has to be transported to all the living cells,
especially to actively growing cells and the cells which stores food.