16 



BOTANY 



PART I 



shoot witli its leaves arranged alternately in two rows shows some- 

 what different relations of symmetry. The diagram of such a shoot 



Fk;. 13a. — Diagram showing the so-called de- 

 cussate arrangement of leaves. 



Fi(i. 13b. — Diagram showing two-ranked 

 alternate arrangement of leaves. 



(Fig. 13b) can only be divided into similar halves by two planes. 



When such a condition exists, a member or plant is said to be 



BILATERAL. When, however, a division 

 into two similar halves is only possible in 

 one plane, the degree of symmetry is in- 

 dicated by the terms dorsiventral or 

 ZYGOMORPHic ; since, while the right and 

 left halves correspond to one another, 

 difterences exist between the dorsal and 

 ventral surfaces. Ordinary foliage -leaves 

 exhibit this dorsiventral structure. In 

 the accompanying figure (Fig. 14) such a 

 monosymmetrical, dorsiventral foliage -leaf 

 is diagrammatically represented. From 

 the surface view (A) and from the cross- 

 section {B), in which the distinction be- 

 tw^een the dorsal and ventral sides is 

 indicated by shading, it is obvious that 

 but one plane of symmetry (.s) can be 

 drawn. Dorsiventral members are often 

 asymmetrical, not being divided by any 

 i plane into corresponding halves : the leaves 



of many kinds of Begonia will serve as 



Fi.^ 14.— Diagram of a foliage-leaf, examples of this. In such cascs — and 



i.trr::,-;:::'fs^mm:Sr^^^ the leaf of the Elm may be mentioned 



as another striking example — the asym- 

 metry of the indi\idiial leaf is subordinated to the symmetry of the 

 entire plant. 



B 



^v\ y!nimi\m\ u\, nm 



