LEAVES 



207 



ends of elongated cells which stand so close together that 

 there is no drying air between them. It seems probable, 

 however, that palisade development has more to do with the 

 intensity of light than with drought. Figure 169 shows in 

 a striking way the effect of different light intensities upon 

 the structure of the mesophyll of leaves of the same plant. 

 It has been observed that the chloroplasts are able to assume 

 various positions in cells, in very intense light moving to the 

 more shaded depths of palisade cells, and in less intense 

 light moving to the more ex- 

 ternal regions of the cells. 



Hairs and scales are very 

 common outgrowths from 

 the epidermal cells of leaves 

 and stems (Figs. 170-172), 

 and it is natural to associate 

 them with the idea of pro- 

 tection of some kind. But 

 they are not related to 

 drought or to intense light 

 definitely enough to make it 

 clear that they are a pro- 

 tection against these dangers, 

 hairy plants are characteristic of dry regions, and also that 

 a covering of hairs is an effective sun screen in regions of 

 intense light, but there also are many hairy plants in moist 

 and well-shaded forests. 



Small leaves are also characteristic of dry regions, and it 

 is easy to see that a small leaf exposes a small surface to the 

 drying air and the intense light, but the total leaf exposure 

 on a plant may not be reduced. That the reduction in size 

 holds a direct relation to the dry conditions is evident from 

 the fact that the same plant often produces small leaves in 

 dry conditions and larger ones in moist conditions, but this 

 is more evidently a response to conditions for growth than, 

 an effort to check transpiration. 



FIG. 173. Section through a leaf of 

 Begonia, showing epidermal layers (e) t 

 water storage tissue (w), and the cen- 

 tral cells containing chloroplasts (c). 



It has been suggested that 



