THE SHAPES OF LEAVES. 159 



converse case. Fig. 223 represents a shoot of Strobilanthes 

 glomeratus. Here the leaves are so set on the stem that the 

 inner half of each leaf is shaded by the subsequently-formed 

 leaf, while its outer half is not thus shaded ; and here we find 

 the inner half less developed than the outer half. But the 

 most conclusive evidence of this relation between unsymme- 

 trical form and unsymmetrical distribution of surrounding 

 forces, is supplied by the genus Begonia; for in it we have 

 a manifest proportion between the degree of the alleged 

 effect and the degree of the alleged cause. These plants 

 produce their leaves in pairs, in such ways that the connate 

 leaves interfere with one another, much or little according 

 as the foot-stalks are short or long; and the result is a cor- 

 relative divergence from symmetry. In Begonia nelumbii- 

 folia, which has petioles so long that the connate leaves are not 

 kept close together, there is but little deviation from a bilater- 

 ally-peltate form; whereas, accompanying the comparatively 

 marked and constant proximity in B. pruinata, Fig. 224, we 

 see a more decidedly unsymmetrical shape; and in B. 

 mahringii, Fig. 225, the modification thus caused is pushed 

 so far as to destroy the peltate structure.* 



§ 231. Again, then, we are taught the same truth. Here, 

 as before, we see that homologous units of any order become 



* We may note that some of these leaves, as those of the Lime, furnish 

 indications of the ratio which exists between the effects of individual circum- 

 stances and those of typical tendencies. On the one hand, the leaves borne 

 by these drooping branches of the Lime are with hardly an exception unsym- 

 metrical more or less decidedly, even in positions where the causes of unsym- 

 metry are not in action : a fact showing us the repetition of the type irrespec- 

 tive of the conditions. On the other hand, the degree of deviation from 

 symmetry is extremely variable, even on the same shoot : a fact proving that 

 the circumstances of the individual leaf are influential in modifying its form. 

 But the most striking evidence of this direct modification is afforded by the 

 suckers of the Lime. Growing, as these do, in approximately upright atti- 

 tudes, the leaves they bear do not stand to one another in the way above 

 described, and the causes of unsymmetry are not in action ; and here, though 

 there is a general leaning to the unsymmetrical form, a large proportion of the 

 leaves become quite symmetrical. 



