82 ROOTS AND TUBEROUS STRUCTURES [CHAP.IX. 



It is obvious that these results issuing from 

 cultivation all occur as " acquired characters " 

 by the soma or vegetative structures ; because 

 there are no reproductive cells at all to be 

 affected by the external influences mechanical 

 only in these cases forming the swollen roots. 



With regard to subterranean stems, they 

 may elongate to great lengths, as in sedges and 

 grasses of our sand dunes, or they may terminate 

 abruptly and swell into tubers, as the potato ; 

 and although the former has not been con- 

 verted into the latter, all analogy points to a 

 similar conclusion. That the tuber of a potato 

 is the consequence of growing underground is 

 proved by allowing a shoot to escape into the 

 air. It will then grow into a leafy stem instead 

 of developing tubers ; but tubers are hereditary 

 structures, as new sorts can be raised from seed. 

 Some tubers act for water storage, as of Poa 

 bulbosa ; but then the tubers are only found in 

 dry situations and not in moist ones, showing 

 a strict adaptation to the environment. 



It is easy to convert an aerial stem into a 

 subterranean one by simply burying the end 

 of the shoot, as of hop, mint, periwinkle, etc. 

 When the apex reappears the portion which 

 has developed underground will be found to 

 have taken on all the characters of normally 

 subterranean and hereditary creeping stems. 



