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 from 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 
Fic. 30.—Basaut Part or Potato Pant. 
a, underground branch bearing stem-tuber 6. 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 round 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-like 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 swollen 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 
