256 STRUCTURAL BOTANY 



generally are badly provided with root-hairs, and 

 many species do not form them at all. As a matter 

 of fact they are not wanted. The use of root-hairs 

 is to increase the absorbent surface by which water 

 can be taken up. The greater the transpiration, the 

 greater is the quantity of water needed, and the more 

 does the absorbent surface require to be increased. 

 Now the leaves of most Coniferse have a very thick 

 cuticle, while the stomata are not very numerous and 

 lie sheltered at the bottom of deep depressions. The 

 water-conducting tissue of the leaf is also, as we 

 have seen, but moderately developed. Hence the 

 amount of watery vapour given off in transpiration 

 is comparatively small, and the supply of water taken 

 up is small in proportion. Thus we see why the 

 absorbent surface of the young roots need not be 

 specially large, and so the absence or slight develop- 

 ment of the root-hairs is explained. 



Within the piliferous layer is a wide zone of 

 parenchymatous cortex, limited on the inner side by 

 the endodermis, which immediately surrounds the 

 central cylinder (see Fig. 104). This endodermis 

 has the structure with which we have already become 

 familiar. Its radial and horizontal walls are corky 

 and undulated, and show the characteristic dark line 

 when seen in section (see Fig. 109, en). 



The cylinder of the root in the Fir may be either 

 triarch or diarch. The main-root always has the 

 former structure. We will describe the diarch type 

 which our figures illustrate. The triarch roots do 

 not differ from this except in the number of bundles. 



