THE WHITE LILY 167 



and external surfaces, as shown in Fig. 64. The 

 endodermis serves especially to cut off all communica- 

 tion between the intercellular spaces of the cortex 

 and the interior of the central cylinder. These 

 spaces contain air, and if this air could make its way 

 into the vessels of the wood, the water-current through 

 the latter would soon be stopped Hence the import- 

 ance of the isolating endodermis. 



The cortex is many layers of cells in thickness, and 

 is persistent instead of being thrown off, as in 

 Dicotyledons. Outside this, again, is the absorptive 

 or piliferous layer, which forms the external surface 

 of the root. Many of its cells grow out into uni- 

 cellular root-hairs (see Fig. 65, r.h.), which are the 

 special organs for absorption, taking up water and 

 dissolved salts from the soil. 



It is only, however, the young parts of the roots 

 which absorb food; sooner or later the piliferous 

 layer dies away, absorption ceases, and the root hence- 

 forth becomes merely an organ of conduction, which 

 serves to pass on the water towards the stem. This 

 conducting part of the root requires some kind of 

 protective layer on the outside to guard it against 

 abrasion, or the more serious danger of attack by 

 parasites, and also to prevent possible loss of water 

 if the soil should become dry. The protective 

 function is performed by the outermost layer of the 

 cortex, which soon becomes distinguished from all 

 the inner layers by its cuticularised walls. For a 

 time it has the structure of an endodermis; subse- 

 quently the corky change extends to all its walla 



