62 



THE STEM 



are very well marked, but they bear only small temporary 

 leaves or none at all. The stems are adapted to function 

 as leaves b}' being flattened and by retaining the green 

 color necessary for active foliage. Thus many desert 



53. Opuntia fiUpendula. A Prickly Pear Cactus, and 

 typical desert i)lant, having a thickened stem with 

 green rind, numerous protective spines but no 

 foliage leaves. The roots are partly transformed 

 by tuberous swellings into organs of storage ; when 

 planted they grow, like the thickened roots of the 

 Sweet Potato. 



plants like the Cactuses (Fig. 53) have no foliage leaves. 

 The green rind takes on their function. The total sur- 

 face of these plants is thus very small compared with 

 the surface exposed by a leafy plant of the same bulk, 

 growing in moist climates. The water that the desert 

 plants are able to obtain through their roots in the wet 



