FOLIAGE LEAVES. 395 



such leaves, as well as those of many succulents, are so thick 

 they provide water storage sufficient for the plants, which radi- 

 ate so slowly from their surface. 



766. (4) Elimination of the leaf. Perhaps the most striking 

 illustration of the reduction of leaf surface is in those cases where 



Fiy. 436. 



A "Phylloclade," leaves absent, stems broadened to function as leaves, on the 

 edges numerous flowers are borne. 



the leaf is either completely eliminated as in certain euphorbias, 

 or in certain of the cacti where the leaves are thought to be re- 

 duced to spines. Whether the cactus spine belongs to the leaf 

 series or not, the leaf as an organ for assimilation and trans- 

 piration has been completely eliminated and the same is true 

 in the phylloclades. The leaf function has been assumed by 

 the stem. The stem in this case contains all the chlorophyll; 

 is bulky, and provides water storage. 



III. Protective Positions. 



767. In many cases the leaves are arranged either in relation 

 to the stem, or to each other, or to the ground, in such a way 

 as to give protection from too great radiation of heat or moisture. 

 In the examples already cited the imbricated leaves of cassiope, 



