HOW PLANTS FEED SAL 
water is evaporated from the leaves, the liquid remaining 
in the leaf-cells becomes thicker than before, and hence 
thicker than that in the neighbouring cells. The denser 
liquid in the outer leaf-cells draws the thinner liquid from 
the inner cells ; and these from cells further back, and so the 
process of diffusion gradually extends through the branches 
and stem to the roots. It is in fact, partly owing to 
transpiration that the force is developed which brings up 
water from the soil. 
Rate of Transpiration. The rate at which a plant 
transpires varies with the temperature; the warmer and 
drier the air is, the more rapid is the transpiration. These 
facts explain various familiar phenomena. On a warm day 
the leaves of many plants, ¢.g., those of the potato and 
pumpkin, are often noticed to hang down and droop. 
This is because the water carried away by transpiration 
is in excess of that absorbed by the roots. Towards 
evening as the atmosphere becomes cooler, the rate of 
transpiration is checked and the leaves return to their 
normal condition. Again, if we place cut flowers in a box 
and exclude the hight, they will remain fresh for a much 
longer time than if exposed to the air. Inside the box 
transpiration is slower because the air quickly becomes 
moist owing to the absence of light. 
Plants Transpire chiefly through their Leaves. 
This can be readily shown by comparing the rate at which 
two branches transpire, one having many and the other 
having few leaves. If the comparison be made by carefully 
weighing the subjects of the experiment as detailed on page 
28, it will be found that more moisture has passed through 
the plant having the greater number of leaves. Again, if 
the leaves be entirely removed from a ‘plant, transpiration 
is very greatly retarded, if not entirely stopped. From 
these experiments we conclude that the leaves, exposing as. 
