72 NATURE TEACHING 



Transpiration. 



Everyday experience shows us that if a leafy shoot 

 is picked it soon becomes limp and then withers, but 

 that if we place it in water it remains fresh and stiff for 

 a longer time. Further, we know that a shoot which 

 has commenced to wither can often be made fresh 

 again by placing the cut end of its stalk in water. 

 Similarly, plants growing in the ground droop and may 

 die if they are deprived of water for a long time. They 

 soon revive if water is poured on the soil so as to 

 penetrate down to their roots. From these various 

 facts it is clear that the withering and limpness of the 

 leaves is due to the fact that they give off water, and 

 that more can be supplied to them either by putting 

 the cut end of the stalk in water, or, as happens in 

 nature, by water being taken up by the roots and 

 passed on through the stem to the leaves. 



This loss of water by the leaves is known as tran- 

 spiration^ and is of great importance to the plant, 

 because as water is given off from the leaves more is 

 steadily drawn up through the stem to take its place. 

 When a plant is growing and has plenty of water at its 

 roots, water is taken up almost as quickly as it is given 

 off, and the whole plant remains fresh ; but if there is 

 none or only very little water to be obtained, as in the 

 cut shoot or the plant in dry ground, the roots cannot 

 take up enough to make up for what the leaves give off, 

 and first the leaves and afterwards other parts of the 

 plant droop and wither. 



In transpiration, the green spongy tissue of the leaf 



