THE LEAF 117 



stand by these first of all the hairs, which in their simplest 

 forms are conical or tubular protrusions of the epidermal 

 cells, as we have seen in the case of the root-hairs. Just as 

 in the first stages of development, some cells of the epidermis 

 remain small and meristematic, and give rise by dividing to the 

 guard cells of the stomata, while others remain tubular, and 

 constitute the ordinary epidermal cells, so some other cells 

 which will not become guard cells also remain meristematic, 

 and retain the power of growing out, dividing and branching, 

 and represent the organs known as hairs. Sometimes a certain 

 connection may be observed between the hairs and the stomata ; 

 in some petals, for instance, the formation of hairs increases 

 as the formation of stomata ceases. 



In the case of the formation of hairs, the plant again pro- 

 ceeds on the lines of increasing its superficial area. As long 

 as the wall of these outgrowths is thin and not cuticularised, 

 they' will be able to absorb water and gases, and also to give 

 them off; they will therefore be able to serve the purpose of 

 transpiration. Later on, when the trichomes have grown to 

 their full size, and have begun to form firm, cuticularised 

 hairs, stellate, glandular, or woolly hairs, their function is a 

 different one. They now represent a covering to the organ, 

 and in the interstices of this covering they hold fast, very fast 

 indeed, a certain quantity of air. It is only with great diffi- 

 culty that the air can be driven out of such a felting of hairs, 

 and it therefore is a very considerable protection against 

 variations of temperature and of moisture. 



Cold will take a long time to affect a leaf covered with such 

 a felted mass of hairs, and rain will not be able to drive out 

 the air from this covering, and will therefore not wet the leaf. 

 But the greatest protection which such a layer of hairs affords 

 is that it will greatly reduce the amount of transpiration from 

 the leaf surface. The wind which passes over a bare thin- 

 walled leaf carries away a great deal of water vapour, while 

 it will scarcely set the air contained between the dense 

 hairs in motion. The amount of transpiration, however, does 

 not depend entirely upon the dryness and the movement of 

 the air, but also largely upon the amount of light. The more 

 intense the illumination, the greater is the evaporation. But 



