THE SHAPES OF LEAVES. 



153 



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 deviate 

 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-chest 

 nut, Fig. 205, already instanced as showing how the arrange 

 ments and sizes of leaflets are determined by the incidence of 

 forces, serves also to show how the incidence of forces deter- 



2.07 



210 



mines the relative sizes and arrangements of leaflets. Fig. 

 210, which shows a leaf of the Bombax, further illustrates 

 this relation of structure to conditions. 



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 

 opposite sides of the main foot-stalk or mid-rib. On turning 

 back to Fig. 65, it will be seen that the compound leaf there 



