n8 



RELATIONSHIPS OF SYMMETRY 



being shut off from the substratum, must remain for the present undecided. 

 The peltate form of the leaf allows of the substratum all round being used, 

 and protects the stem and its buds. 



Reverting to the asymmetric leaves l we have to remark as follows 

 regarding the disposition of the sides of the leaf: 



The disposition of the sides of the leaf is not always the same in the 

 species of one genus. In the case of a climbing shoot with two rows 

 of leaves and the leaf-laminae all in one plane, as for example in 

 Pellionia (Fig. 62), the large sides in one plant may be turned to 

 the apex of the shoot, in another they may be turned away from 

 it. The genus Begonia has been somewhat carefully examined, and 

 I shall discuss its whole relationships of growth in a few examples. 



FlG. 70. Begonia incarnata, and others. Scheme 

 of the formation of the leaves and the branching seen 



from in front 



FIG. 69. Begonia Rex. Scheme of the arrangement of the leaves and the branching seen from in front. 

 f'F"root-side of the shoot-axis which is shown in cross-section above W. In front of each leaf are two stipules seen 

 in cross-section, whilst the leaves themselves are shown in surface-view. In the axil of the second leaf, apparently 

 in that of the lower stipule, there is an axillary shoot. 



Of course I can only deal with the more general dispositions, for the 

 configuration and the biological relationships of the species of Begonia 

 are manifold. 



The genus is distinguished by producing exclusively dorsiventral 

 shoots in the vegetative region, and this dorsiventrality expresses itself 

 in the form of the leaves and of the stipules, and also in many species 

 in the position of the leaves and lateral shoots. All species of Begonia 

 have a distichous phyllotaxy and more or less asymmetric leaves, but 

 there are many variations. If we look from the front at a shoot 

 of a Begonia which has grown obliquely, placing it in a horizontal position 

 so that its under side is accurately below, and its upper side is directed 



1 I may remark here that it does not follow that light is the sole exciting factor in asymmetry of 

 leaves because the phenomena stand in relation to light; what I have said above regarding cotyledons 

 shows that various external factors may induce asymmetry. 



