TISSUES AND SIMPLE ORGANS. 141 



tliis name suggests its similarity to tlie protective slieath (endoderm) 

 of ordinary roots. In it are found passages to the internal paren- 

 chyma (Leitgeb) ' which no doubt serve to conduct moisture to the 



Fig. 81. — Cross-section of a root with root-hairs. 



(After Frank.) 



interior. The outermost cells of the velamen may develop hair- 

 like structures, especially when the growing root lies in contact 

 with a solid moist body. 



IX. THE APPROPRIATION OF ASSIMILATED FOOD- 

 SUBSTANCES. 



In order to form a correct conception of the processes of nutri- 

 tion which are about to be considered it is necessary to have a clear 

 understanding of carbon-assimilation (see pp. 128 et seq.). It is 

 true that the mass of solid food-sul)stances and increase in the 

 weight of plants can be traced to the disintegration of atmos- 

 pheric CO, by the green organs ; but plant-life in its various con- 

 ditions and conformations presents a series of phenomena which 

 occur as regularly as the process of assimilation, and which teach us 

 that the appropriation of food-substances already assimilated is an 



1 This investigator (1864) made a special study of the aerial roots of orchids. 



