634 ECOLOGY 



clearly is associated with a relative surplus of water, while succulence commonly 

 is associated with a relative surplus of salts. The best conditions for active growth 

 border closely on those which induce intumescence, water being present in sufficient 

 amount to make the cell sap dilute and thus permit unrestricted cell activity. In- 

 deed, the phenomena of edema mean that growth within the leaf is so rapid that 

 expansion fails to keep pace with it. Succulence, on the other hand, results from 

 sluggish growth, the water, because of its large percentage of solutes, being utilized 

 less freely. When exposed to transpiration, the intumescent plant withers quickly, 

 while the succulent plant long retains its moisture, owing to the large amount of 

 solutes. However, not all cases of leaf succulence are to be explained thus. In 

 some cases the cell sap is known to be dilute, and in many instances its condition is 

 yet to be investigated. Still less is known as to the factors involved in the origin of 

 concentrated cell sap, except, perhaps, in halophytes. It has been suggested that 

 the respiration of succulent plants is imperfect, thus leading to the accumulation of 

 osmotically active substances which facilitate further succulence ; it is difficult, 

 however, to understand the inception of such a process. Still more inexplicable is 

 succulence in such plants as Begonia and Peperomia, which have fleshy leaves in 

 the humid atmosphere of the rain forest. 



The advantages of succulence. — General advantages. — Even though 

 a succulent plant is unwatered for a long time, the grovi^ing parts show 

 no cessation of activity, new shoots continuing to develop by utilizing 

 the water in the older organs. The old leaves shrink and become 

 wrinkled, the cells collapsing to a greater or less degree. The func- 

 tional significance of leaf succulence is to be found chiefly in the pro- 

 tection of the synthetic tissue. Leaves are in general ill-fitted for water 

 retention, since they are much more subject to excessive transpiration 

 than are stems and roots. Furthermore, leaf succulence must impair 

 synthetic activity, partly because of the weakening of incident light in 

 traversing the leaf, and partly because the accumulation of solutes im- 

 pairs sugar formation and other cell activities. However, in extreme 

 xerophytes it is not a question between ample and slight synthetic 

 activity, but between slight actinty and none at all ; the very presence of 

 abundant water and solutes, though tending generally to reduce syn- 

 thesis, may here be the means of permitting synthesis through the 

 protection thus afforded to the chlorenchyma. Such protection may 

 vary with the relative arrangement of the water tissue and the chloren- 

 chyma. Whether its position is peripheral or central, water passes from 

 the water tissue to the chlorenchyma, whenever the supply in the latter 

 becomes scanty, thus permitting the continuance of synthetic activity. 

 Peripheral water tissues also may tone down the intensity of the inci- 

 dent light, thus permitting synthesis to continue even in bright sunlight. 



