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 

 F iG.46.-parto(aPota,oPia n t. presented by a 



The dark tuber in the middle is the one from which plant of given 

 the plant has grown. 



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, 



