78 FOUNDATIONS OF BOTANY 
tulip, and the skunk-cabbage, owe their early-blooming 
habit to richly stored underground stems of some kind, 
or to thick, fleshy roots. 
92. Condensed Stems.— The plants of desert regions 
require, above all, protection from the extreme dryness of 
the surrounding air, and, usually, from the excessive heat 
of the sun. Ac- 
cordingly, many 
desert plants are 
found quite desti- 
tute of ordinary 
foliage, exposing 
to the air only a 
small surface. In 
the melon-cactuses 
(Fig. 49) the stem 
_ appears reduced 
to the shape in 
which the least 
possible surface is 
presented by a 
Fig. 46.— Part of a Potato Plant. i 
The dark tuber in the middle is the one from which plant of given 
the plant has grown. bulk, —_ that is, in 
a globular form. Other cactuses are more or less cylindri- 
cal or prismatic, while still others consist of flattened 
joints; but all agree in offering much less area to the sun . 
and air than is exposed by an ordinary leafy plant. 
93, Leaf-Like Stems.—The flattened stems of some kinds 
of cactus (especially the common, showy Phyllocactus) are 
sufficiently like fleshy leaves, with their dark green color 
and imitation of a midrib, to pass for leaves. There are, 
