LEAVES 



605 



appear better nourished than do the other leaves. These observations 

 would suggest that the lobed leaf represents a sort of skeleton, 

 which, under favorable vegetative conditions {e.g. freedom from ex- 

 cessive transpiration or good food supply), may become filled out into 

 an entire leaf. 



This view, perhaps, is supported by the fact that various leaves (as in Ricinus) 

 are more divided in youth than at maturity, the regions between the principal veins 

 developing last. If such leaves while young were exposed to higher transpiration 

 or to decreased food supply, they probably would remain in the more lobate condi- 

 tion. In the mulberries the more lobate leaves usually are small and on slender 



Figs. 887, 888. — Leaf variation in the Japan ivy {Pseiera tricuspidala) ; 887, a three- 

 lobed simple leaf from the upper portion of a vine ; 888, a compound leaf with three leaf- 

 lets from the lower portion of a vine. 



branches (as if poorly nourished), while the simple leaves commonly are larger 

 and on more luxuriant shoots (figs. 882-884). Early in the year the leaves of the 

 staminate tree may be larger and more entire than are those of the pistillate tree, 

 perhaps because in the latter the food otherwise available for leaf construction is 

 utilized in fruit development. In the barberry there are gradations between 

 ordinary leaves and leaf skeletons which are reduced essentially to spines (figs. 

 885, 886), and it is possible that the latter are due to poor nutrition or perhaps to 

 excessive transpiration (see discussion of stem spines, p. 741). 



While the compound air leaves thus far considered diflfer from divided 

 water leaves in appearing where transpiration is high, rather than where 

 it is low, in each case their development seems to be associated with poor 

 nutrition. The lack of rigorous experimental data forbids further analy- 

 sis, and makes dubious even the suggestions here given. 



While the Campanula and Geum categories of leaf variation are poorly 

 enough understood, chiefly, perhaps, because of inadequate experi- 

 mentation, there are many cases of changing form that are not under- 

 stood at all. In a number of plants (as Lepidium, Lactuca) the large, 



