144 MORPHOLOGICAL DEVELOPMENT. 



converse case. Fig. 223 represents a shoot of GoMfussia 

 glomerata. 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 

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

 aiost conclusive evidence of this relation between unsynuue- 

 fcrical 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 a way that the connate 

 leaves interfere with one another, mnch or little according 

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

 relative divergence from symmetry. In Begonia nelumbife- 

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

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

 rally-peltate form; whereas, accompanying the compara- 

 tively marked and constant proximity in B. pruinata, Fig. 

 224, we see a more decidedly unsymmetrical shape ; and in 

 B. mahringX, 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, famish indica- 

 tions of the ratio which exists between the effects of individual circumstances and 

 those of typical tendencies. On the one hand, the leaves borne by these drooping 

 (tranches of the Lime are with hardly an exception unsymmetrical more or lea 

 decidedly, even in positions where the causes of unsymmetry are not in action : a 

 feet showing us the repetition of the type irrespective 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 

 highly 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 attitudes, the leaves they bear do not stand to one 

 another in the way above described, and the causes jf 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 



