IX] STEM-TUBER 123 



Stem-tubers of the same kind are found in the South 

 American plant Ullercus tuberosus, a member of quite a 

 different family, the Jerusalem Artichoke, Stachys tubifera, 

 &c. } but in the last two cases the swollen bud composing 

 the tuber is not on a long stolon. These cases, and their 

 relationships, become very clear on comparing the short, 

 swollen, scale-bearing underground runners of Smithi- 

 antha zebrina, and the similar super-terrestrial ones of 

 Kohleria digitaliflora and other Gesneraceae, with the 

 tubers of Stachys tubifera and the potato. In Apios 

 tuberosa, a North American leguminous plant, several 

 but not all of the internodes of the stolon swell to tubers, 

 in Vitis gongylodes only one or two. each of which, there- 

 fore, is devoid of " eyes " on its sides : in Nelumbium 

 luteum one internode only of a short stolon thus swells, 

 and has only a terminal bud. In the Black Bryony 

 (Tamus communis) and other Dioscoreae, the lowermost 

 internode above the cotyledon swells as a tuber about 

 the level of the soil ; and in Testudinaria this grows larger 

 year by year until it attains enormous dimensions, throw- 

 ing up new stems annually from its terminal buds. In 

 Bowiea we have also a perennial basal corm. In the 

 cases last mentioned, where the tuber consists of swollen 

 parts of the main axis, we have the key to an understand- 

 ing of several peculiar modifications of stems. It is clear 

 that such tubers are distinguishable by no sharp lines 

 from the short erect rhizomes called " root-stocks," and in 

 some Gesneracese (e.g. Gorytholoma splendens) the whole 

 rhizome becomes a large irregular tuber, and similarly in 

 many Basellaceae. The next example will show that 

 these cases are equally closely related to what are termed 

 Corms. 



The common Arum has a very short, horizontal swollen 

 rhizome (root-stock) which ends in the bud that developes 



