6 



PART I. MORPHOLOGY. 



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however, essentially similar. When, however, a body is divisible in at 

 least two planes into precisely similar halves, it is said to be polysym- 

 metrical. 



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

 bilaterally symmetrical when it presents an interior, a posterior, 

 and two lateral surfaces ; the lateral surfaces, or flanks, being dif- 

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

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

 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. 



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 sym- 

 metrical ; k carpel. B with two planes of symmetry, is isobilateral ; / the suture ; * the 

 seed. C with a single plane of symmetry, is zygomorphic and dorsiventral ; R dorsal 

 surface ; B ventral surface ; r right, and I left flank ; fc stone. 



a. Isobilateral Symmetry. Isobilateral symmetry is usually 

 manifested 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 B) shows that internal, as 

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



