104 TRANSPIRATION. [CH. VII 



in response to changes in the environment of the plant. 

 The mechanism by which they do so need not be described, 

 it must suffice us to know that when the plant begins to 

 suffer from want of water, or when it is exposed to some 

 other conditions, e.g. darkness, the cleft between the guard 

 cells, and therefore the passage from the outer air to the 

 intercellular spaces of the leaf, is closed. The most 

 important function of the stomata is the aeration of the 

 inner parts of the leaf. And since it is through the stomata 

 that the C0 2 enters the leaf, these organs are of great 

 importance in the nutrition of the plant. But they 

 also influence the degree to which the leaf loses water by 

 evaporation a function known as transpiration. 



Transpiration. 



If a delicate leaf is gathered on a hot, dry day, it 

 withers almost immediately, that is, its cells collapse for 

 want of water in the manner already described. This 

 shows two things, (1) that leaves are constantly losing 

 water by evaporation, (2) that, since the leaf does not 

 wither if left on the tree, the loss of water is, under 

 normal circumstances, continually made good. 



The means by which a current of water is earned from 

 the root to the top of a high tree is still an unsolved problem. 

 There are however certain fundamental experiments which 

 must be considered. 



If a branch, such as that of a laurel, be cut and placed 

 in a coloured fluid, e.g. eosin dissolved in water, and left 

 there for some hours, it will be seen that the coloured 

 fluid has travelled up it, showing that there is a sucking 



