4. SYMMETRY. 7 



obtain in all cases ; the internal structure of isobilateral leaves is 

 often not strictly isobilaterally symmetrical. 



b. Zygomorphic Symmetry. A zygomorphic or rnonosymmetri- 

 cal body or member is divisible into two similar halves in one 

 plane only (Fig. 2 (7). Of this there are two principal cases : 

 First, that in which the anterior and posterior halves are similar, 

 whilst the right and left halves are dissimilar, in other words, 

 when the plane of symmetry is lateral ; the body or member is 

 then laterally zygomorphic (e.g. flower of Corydalis) : secondly, 

 that in which the anterior and posterior halves are dissimilar, 

 whilst the right and left halves are similar, in other words, when 

 the plane of symmetry is antero-posterior ; the body or member is 

 then said to be dorsiventrally zygomorphic, or, briefly, dorsi- 

 ventral. Less frequently, as in some flowers, the plane of 

 symmetry is neither lateral nor antero-posterior, but intermediate 

 between the two, the zygomorphy being oblique. 



Of these possible forms of zygomorphic symmetry, the dorsi- 

 ventral is the most common. The term is derived from the use of 

 the terms dorsal and ventral to indicate, respectively, the dis- 

 similar anterior and posterior halves of the body or member. 



The application of the terms dorsal and ventral to the two dissimilar 

 halves of the body or a member requires some explanation. Generally 

 speaking, the under surface of a dorsiventral body is the ventral, the 

 upper the dorsal. In the case of leaves, however, the terms dorsal and 

 ventral are used with reference to the parent stem : the upper or inner sur- 

 face is here the ventral, the outer or lower, the dorsal. 



The difference between the dorsal and ventral halves may be 

 exhibited in their external form. Thus, the dorsal and ventral 

 halves of many fruits (peach, Fig. 2 C ] or a pea-pod) may be 

 distinguished at once by their form. Or the difference may be in 

 the nature of the members which they bear ; thus, creeping dorsi- 

 ventral shoots commonly bear (adventitious) roots or root-hairs on 

 their ventral, and branches or leaves on their dorsal surface ; or 

 the one surface may bear lateral members, and the other none. 

 Or, finally, the difference may exist in their internal structure ; 

 thus, in dorsiventral foliage-leaves, the internal structure of the 

 dorsal half is different from that of the ventral half. 



It must not be overlooked that the terms radial, isobilateral, and 

 dorsiventral, may be all applicable to one and the same body or 

 member, according to the particular feature which is taken into 

 consideration. For example, a branch of the Silver Fir is, in its 



