29 



Stem-tubers of the Jerusalem Artichoke (Helianthus tuberosus). 



species of Dioscorea yielding " yams." The tubers of the terrestrial 

 Orchids are chiefly composed of radical structures. If we examine the 

 twin tubers of Orchis Morio (fig. 21), we find one at the base of the 

 flowering stem (a), which towards the close of the season is withered, 

 while the other (b), crowned by a bud (*), is solid and healthy : in the 

 axil of the lowest leaf of this bud exists another bud in a rudimentary 

 state ; and as the oldest tuber shrivels, this swells out and assumes its 

 form, in the next season appearing as the bud-tuber, while its parent 

 becomes the tuber of the flowering stem. The greatest part of the 

 mass of these tubers consists of a swollen adventitious root, which is 

 intimately blended with a few little-developed stem-internodes and the 



Fig. 21. Fig. 22. 



Double root- tubers of Orchis Morio: 

 a, old tuber ; 6, new tuber with the 

 bud * for the next season. 



Double palmate root-tubers of Gymnadenia 

 odoratigsima : a, old tuber at the base of 

 the old flower-stem ; b, new tuber, with 

 bud for the next season. 



