144 MORPHOLOGICAL DEVELOPMENT. 



converse case. Fig. 223 represents a slioot of Goldfussia 

 glomerata. Here the leaves are so set on tlie stem that the 

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

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

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

 Host conclusive evidence of this relation between unsjomne- 

 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 a way 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 symmetrj^. In Begonia iielmnbicB- 

 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 unspnmetrical shape ; and in 

 B. mnhnngii, 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 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 

 Qranches of the Lime are with hardly an exception unsymmetrical more or less 

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

 fact 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 staking evidence of thia 

 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 Avay above described, and the causes Df unsymmetry are not in 

 action ; and here, though there is a general leaning to the unsj'mraetrical form, 

 a large proportion of the leaves become quite symmetrical 



