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 aU, 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 

 plant of giren 

 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, 



Fig. 46. —Part of a Potato Plant. 



The dark tuber in the middle is the one from which 



the plant has grown. 



