49 SOUTH AFRICAN BOTANY 
(b) The tuber (fig. 26) is a portion of a stem growing 
underground and swollen by the deposition of food 
material. It differs from a rhizome in that it is only a 
portion of astem, instead of a continuous stem. A potato 
is a good example of a tuber. It bears modified leaf 
buds called “eyes”. These are inserted in a similar 
order to the leaves on the aerial part. Tubers form a 
valuable method of propagating plants, and this is the 
chief means of propagation in the case of the potato 
plant and Jerusalem artichoke. 
Fic. 26.—Tubers of Potato. 
(c) The bulb (fig. 27) consists of a flattened disc-like 
portion of the stem, surrounded by the swollen bases 
of leaves. The outside is often protected by thin 
membranous leaves. There are two kinds of bulbs, 
tunicated bulbs (in which the outer leaves are large and 
completely ensheath the inner leaves), e.g. onion, and 
scaly bulbs, in which all leaves are about the same size 
and overlap each other, e.g. tulip. In the interior of the 
bulb in the axil of one of the fleshy leaves is a bud 
which grows in spring at the expense of the food stored 
