16 



BOTANY 



their longitudinal axis. The degree of symmetry peculiar to any 

 leafy shoot will be more apparent from a diagram, that is if the 



KIC;. lli. -Diagram showing tin- su-callfl cl> 

 rnssiiti- arrangement <>t' leaves. 



. 14. Diagram showing two-tmoked 



alternate arrangement of lea\ 



leaves which it bears be projected on a plane at right angles to its 

 axis. The radial symmetry of a shoot with opposite leaves is clearly 



shown in the adjoining diagram (Fig. 13). 

 I lV A shoot with its leaves arranged alternately 



in two rows shows somewhat different rela- 

 tions of symmetry. The diagram of such 

 a shoot (Fig. 14) 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 sym- 

 metry is indicated by the terms DORSI- 

 VENTRAL or ZYGOMORPHIC; since, while the 

 right and left halves correspond to one 

 another, differences exist between the dorsal 

 and ventral surfaces. Ordinary foliage- 

 leaves exhibit this dorsiventral structure. 

 In the accompanying figure (Fig. 15) such 

 a monosymmetrical, dorsiventral foliage- 

 leaf is diagrammatical ly represented. From 

 the surface view (A) and from the cross- 

 section (/?), in which the distinction between 

 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 plane 

 into corresponding halves : the leaves of many kinds of Begonia will 



.v 



<i. 15. Diagram of a foliage-leal. 

 .1, Surface view ; li, trausverse 

 section : -. plane of symmetry. 



