10 PART I. THE MORPHOLOGY OF PLANTS. [ 2. 



one plane only; when it is so divisible in two planes, the halves 

 resulting from the section in one plane, are different from the 

 halves resulting from section in the other. 



Bilateral members are distinguished as isobilateral or as zygo- 

 morphic (or monosymmetrical), accordingly as they are symmetri- 

 cally divisible in two planes or in one plane. 



a. Tsobilateral Symmetry. Isobilateral symmetry is usually mani- 

 fested in the external form. Thus, a walnut is at once seen to be 

 divisible into two symmetrical halves by section, either through 

 the suture, or at right angles to this plane (Fig. 2, B) ; so also a 

 flattened erect leaf like that of the Iris. 



It may be manifested by the position of the lateral members ; for 

 instance, in many shoots (e.g. the Elm) the leaves are borne in two 

 rows, right and left, one row on each flank. 



It may be manifested also in the internal structure. Thus, a 

 transverse section of a walnut (Fig. 2 S) shows that internal, as 

 well as external, isobilateral symmetry exists. But this does not 

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

 often not strictly isobilaterally symmetrical. 



b. Zygomorphic Symmetry. A zygomorphic or monosymmetri- 

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

 plane only. 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 dorsiventral. Less fre- 

 quently, as in some flowers (see page 508, Symmetry of the Flower), 

 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 dissimilar 

 anterior and posterior halves of the body. 



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 dorsiventi al body is the ventral, the upper the dorsal. In 

 the case of leaves, however, the terms dorsal and ventral are used with reference 



