142 PLANT LIFE 



cactus, often grown for the sake of their 

 beautiful flowers, bear a strong resemblance 

 to lobed, fleshy leaves. They are, however, 

 merely stems, flattened by the action of 

 light, which is probably indirectly operative 

 through nutritive processes. If the phyllo- 

 cacti are grown in the dark they only produce 

 rod-like stems, very different from the leaf- 

 like shape assumed under ordinary conditions 

 of illumination. 



The cacti furnish a remarkable range of 

 forms. All of them are pronounced xero- 

 phytes. Now it is a curious fact that these 

 plants of the Western hemisphere have their 

 doubles in some of the euphorbias, or spurges, 

 of the hot, dry regions of the Eastern tropics. 

 So faithfully, indeed, are many of the leading 

 types of cactus forms reproduced, that an 

 untrained observer may easily be deceived. 

 No satisfactory explanation has been given for 

 the occurrence of these closely similar forms 

 of plants, which are widely sundered in 

 affinity and properties, as well as in their 

 geographical distribution. 



The ordinary winter buds of our trees and 

 shrubs, with their protective scales, provide 

 us with still other adaptations by which loss 

 of water from the enclosed leaves is prevented. 

 The young leaves, since they are not only very 

 thin, but are imperfectly protected by a cuti- 

 cular surface, would assuredly suffer if they 

 were exposed to the air. This is probably 

 the chief function of the bud scales, though 



