133 



MORPHOLOGICAL DEVELOPMENT. 



development of the leaflets on the side next the axis is not 

 much hindered. Still the interference of the leaves with one 

 another is, on the average, somewhat greater on the proximal 

 side than on the distal side ; and hence the interior leaflets 

 are rather less than the exterior leaflets. In further proof of 

 which influence, let it be added that, as shown in the figure, 

 at a, the leaves growing out of the flowering-stem devi- 

 ate towards the two-sided form more decidedly. Two- 

 sidedness is much greater where there is a greater relative 

 proximity of the inner leaflets to the axis, or where the foot- 

 stalk approaches towards a horizontal position. The Horse- 

 chesnut, Fig. 205, already instanced as showing how the 

 arrangements and sizes of leaves are determined by the 

 incidence of forces, serves also to show how the incidence 

 of forces determines the relative sizes and arrangements 

 of leaflets. Fig. 210, which shows a leaf of the 



BombaXj further illustrates this relation of structure to con- 

 ditions. 



Compound leaves that are completely bilateral, present us 

 with modifications of form exemplifying the same general 

 truth in another way. In them the proximal and distal 

 parts have none of that resemblance which we see in those 

 intermediate forms just described : the portion next the axis 

 and the portion furthest from the axis are entirely different ; 

 and the only likeness is between the wings or leaflets on 



