STEMS 



61 



the skunk-cabbage, owe their early-blooming habit to richly- 

 stored underground stems of some kind, or to thick, 

 fleshy roots. 



77. 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. Accordingly many desert 

 plants are found quite destitute of 

 ordinary foliage, exposing to the air 

 only a small surface. In the melon- 

 cactuses (Fig. 81) the stem appears 

 reduced to the shape in which the 

 least possible surface is presented by 

 a plant of given bulk, — that is, in a 

 globular form. Other cactuses are 

 more or less cylindrical 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. 



78. Leaf -like Stems. — The flattened 

 stems of some kinds of cactus (espe- 

 cially 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, however, a good many cases in 

 which the stem takes on a more strikingly leaf-like form. 

 The common asparagus sends up in spring shoots that bear 

 large scales which are really reduced leaves. Later in the 

 season, what seem like thread-like leaves cover the much- 

 branched mature plant, but these green threads are actually 



sea 



Fig. 35. Longitudinal 

 Section of an Onion 

 Leaf. 



sea, thickened base of 

 leaf, forming a bulb- 

 scale ; s, thin sheath 

 of leaf; bl, blade of 

 leaf; int, hollow in- 

 terior of blade. 



