LEAVES 199 



so much as the direction in which it moves. The importance 

 of this mass movement of water not only consists in making 

 good the loss through transpiration and thus insuring the 

 continuation of transpiration, but also it carries to the work- 

 ing cells the soil materials especially necessary in the manu- 

 facture of proteins. 



123. Exposure to light. It is evident that leaves should 

 be so adjusted as to receive as much light as possible without 

 danger, for too intense light is dangerous. It is a problem 

 of " just enough and not too much." The adjustment to 



FIG. 163. Rosette of mullein (A) and of evening primrose (B). 



light, therefore, is a delicate one, for the exact position any 

 particular leaf holds in relation to light depends upon many 

 circumstances, and cannot be covered by a general rule, 

 except that it should get all the light it can without danger. 



The ordinary plane of a leaf is approximately horizontal 

 (Fig. 161), a position which enables it to receive the direct 

 and most intense rays of light upon its upper surface. Cer- 

 tainly in this position more rays strike the leaf than if its 

 plane were oblique or vertical, the latter position often being 

 spoken of as an " edgewise " position. Most leaves when 

 fully grown are in a fixed position that has been determined 

 by the conditions that prevailed during their development. 



