LONGEVITY OF PLANTS 111 



stem or root. In the potato (Fig. 30) the tubers are the 

 swollen tips of underground stems, and the " eyes " upon 

 them are the buds. In the dahlia some of the roots 

 which grow frpm the base of the stem become tuberous. 

 In the lesser celandine and some orchids, buds at the 

 base of the stem develop an adventitious root which 

 becomes large and tuberous. The fleshy tuberous roots 

 of biennials are generally complex structures. The part 

 from which the buds arise is stem. In the carrot and 

 turnip the greater part of the tuber is derived from 

 hypocotyl, that portion of the axis that lies between the 

 primary root and the cotyledons ; the lower part, bearing 

 rootlets, represents the primary root. In the swede 



Fio. 30. — Basai Pakt of Potato Plant. 



a, undorground branch bearing stem-tuber 5. To the left branches of an 

 aerial shoot are producing tubers. 



(Fig. 31) the stem forms a collar at the top of the tuber ; 

 most of the rest is hypocotyl. In the kohlrabi (Fig. 32) 

 (German, cabbage-turnip) a big roimd tuberous body 

 projects from the soil, the surface of which is covered 

 with leaf-scars ; this is stem. In the dandelion and 

 chervil the carrot-hke tap-roots are perennial. In the 

 false oat {Arrhenatherum avenaceum) the perennating 

 function is assumed by the base of the stems, the inter- 

 nodes of which become swoUen with food, forming chains 

 of little tubers (Fig. 33). 



3. Bulbs and Corms. — These are compressed buds, or 

 rhizomes, capable, by reason of the food they contain, of 

 living an independent life. Corms are solid bulbs, the 

 food being stored in a short tuberous stem which is covered 



