54 BOTANY 



little below the surface of the leaves, being thus covered 

 only by a thin transparent layer of cells. The dis- 

 tribution of these green bodies, which are known as 

 chloroplasts, so bears a very definite relation to the 

 incidence of the light, and suggests to us that while one 

 duty of the leaf is to exhale watery vapour, another 

 is to secure the illumination of a definite part of its 

 mechanism, which is concerned with the most intimate 

 questions of nutrition. 



As the two functions thus suggested are found upon 

 further inquiry to be intimately bound up with the well- 

 being of the plant, we must examine them a little more 

 closely before looking for the ways in which they in- 

 fluence the form and position of the shoot system. 



There are two reasons for the copious evaporation of 

 water which we have pointed out. The first is con- 

 nected with the problem of feeding, as we noticed in 

 our introductory chapter. Certain constituents, either 

 entering into the composition of the food itself or 

 necessary factors in its construction, are only to be 

 found in the soil and are procured therefrom by the 

 roots. These compounds are absorbed from the soil in 

 solution in the water entering the root hairs, and the 

 solutions are necessarily very dilute to facilitate their 

 passage through the living substance of the hairs. As 

 with a rapidly-growing plant continuously increasing 

 quantities of these substances are needed for nutritive 

 purposes, it follows that large quantities of the solution 

 must be absorbed. In the plant these mineral com- 

 pounds are taken from the water, and the great bulk of 

 the latter is evaporated into the intercellular passages 

 and the vapour subsequently passed out of the stomata. 

 Hence, speaking broadly, the more water that is taken 

 up and subsequently evaporated, the more mineral 

 matter is secured for the use of the organism. 



But there is another and equally important function 



