PART I. THE MOBPHOLOGY OF PLANTS. 



[2. 



the annual rings are comparatively rarely arranged symmetrically 

 around the geometrical centre. The longitudinal axis, then, is a 

 line passing through the organic centres of the successive trans- 

 verse sections. 



Two kinds of symmetry may be distinguished ; the multilateral, 

 including the radial ; and the bilateral, including the isolilateral 

 and the zygomorpliic. The determination of the nature of the 

 symmetry of a body or member depends upon (a) its external 

 form, (6) the arrangement and form of the members which it may 

 bear, (c) its internal structure. 



1. Multilateral and Radial Symmetry. Absolute multilateral 



symmetry is only pre- 

 sented by a body or 

 member which is 

 spherical and has no 

 distinction between 

 base and apex. For 

 example, the body of 

 Volvox can be divided 

 into symmetrical 

 halves in any plane 

 passing through the 

 centre (Fig. 1.). 



The more limited 

 form of multilateral 

 symmetry, which may 

 be conveniently dis- 

 tinguished as radial, 

 is that which obtains 

 in cylindrical bodies 

 or members. It is multilateral symmetry about the longitudinal 

 axis. In this case the body or member can be divided in various 

 planes along the longitudinal axis into a number of similar 

 halves. 



A mushroom with a central stalk, an apple, a cylindrical tree- 

 trunk, are radially symmetrical as regards their external form. 



As regards the position of the lateral members, the trunks of 

 Firs and Spruces, with branches arising on all sides, are radially 

 symmetrical ; and, as regards the form of the lateral members, 

 the flowers of the Rose and of the Tulip are radial (see page 507, 

 Symmetry of the Flower). 



FIG. I. Volvox Glolator (after Cohn ; x about 100), 

 illustrating multilateral symmetry. 



