THE MORPHOLOGICAL COMPOSITION OF PLANTS. 61 



those plants which produce considerable masses of leaves, 

 a\nce the development of mid-ribs into footstalks, by throw- 

 ing the leaves further away from the axes, will diminish the 

 shading of the leaves, one by another. And then, among 

 plants of bushy growth, in which the assimilating surfaces 

 become still more liable to intercept one another's light, 

 natural selection will continue to give an advantage to those 

 which carry their assimilating surfaces at the ends of the 

 petioles, and do not develop assimilating surfaces close to 

 the axis, where they are most shaded. Whence will result 

 a disappearance of the stipules and the foliar fringes of the 

 mid-ribs ; ending in the production of the ordinary stalked 

 leaf, Fig. 99, which is characteristic of trees. Meanwhile, 

 the axis thickens in proportion to the number of leaves it 

 has to carry, and to put in communication with the roots ; 

 and so there comes to be a more marked contrast between it 

 and the Detioles, severally carrying a leaf each.* 



194. When, in the course of the process above sketched 

 out, there has arisen such community of nutrition among the 

 fronds thus integrated into a series, that the younger ones 

 are aided by materials which the older ones have elaborated ; 

 the younger fronds will begin to show, at earlier and earlier 

 periods of development, the structures about to originate 

 from them. Abundant nutrition will abbreviate the intervals 

 between the successive prolifications ; so that eventually, 

 vvhile each frond is yet imperfectly formed, the rudiment of 

 the next will begin to show itself. All embryology justifies 

 this inference. The analogies it furnishes lead us to expect 

 that when this serial arrangement becomes organic, the 

 growing part of the series will show the general relations of 



* Since this paragraph was put in type, I have observed that in some varieties 

 of Cineraria, as probably in other plants, a single individual furnishes all these 

 forms of leaves all gradations between unstipulated leaves on long petioles, and 

 leaves that embrace the axis. It may be added that the distribution of these va- 

 rious forms, is quite in harmony with the rationale above given. 



