2.] 



CHAPTER I. GENERAL MORPHOLOGY. 



9 



The transverse section usually shows complete radial symmetry 

 'of structure, at least when the member is young; it may become 

 somewhat asymmetrical when older, as in the case of the tree- 

 trunks mentioned above. 



A radial body or member can be divided by radial longitudinal 

 sections in two or more planes, into symmetrical halves, which are 

 to each other as an object and its image reflected in a mirror 

 (in Fig. 2, J, the halves obtained by the sections 1-1, 2-2, 3-3, 

 4-4, 5-5). The possible number of such similar halves is not 

 always the same, but it is in any case at least four. In a mush- 

 room or a fir-stem, there are many possible planes of symmetrical 

 section ; but in a Tulip, the sections being taken through the 

 longitudinal axis of the floral leaves, only three are possible ; and 



FIG. 2. Diagrammatic transverse sections of A. an apple ; B, a, walnut ; C, a peach ; 1-1, 

 5-5, are the planes of symmetry. A, with five planes of symmetry, is radially symmetrical ; 

 /, carpel. B, with two planes of symmetry, is zygomorphic ; /, the suture ; s, the peed. 

 C, with a single plane of symmetry, is dorsiventral; R, dorsal surface ; B, ventral surface ; 

 r, right, and I, left flanks ; Ic, stone. 



in an apple, if they pass through the loculi of the core, only five 

 (Fig. 2 A). 



The two halves are not always as exactly alike as an object and its reflected 

 image ; this is not the case, for instance, in a fir-trunk, because the lateral 

 branches are not borne at the same level. The two halves are, however, essen- 

 tially similar. When, however, a body is divisible in at least two planes into 

 precisely similar halves, it is said to be poly symmetrical. 



2. Bilateral Symmetry. A body or member is said to be bilate- 

 rally symmetrical when it presents an anterior, a posterior, and 

 two lateral surfaces; the lateral surfaces, or flanks, being different 

 from the anterior and posterior. Such a body or member is 

 divisible into two symmetrical halves, either in two planes, or in 



