4 PART I. MORPHOLOGY. [8 4 



The stem, leaf, and root of any one plant present the same kind 

 of complexity of structure : the hair is of much simpler structure 

 as a rule. 



4. The Symmetry of the Body and of the Members. 

 Whatever the form of the body or of a member, it has three axes 

 at right angles to each other. When these three are all equal, the 

 body is a sphere (e.g. Volvox, Fig. 1) : when two are equal, and 

 both longer than the third, the body or the member is a flattened 

 circular expansion (e.g. Pediastrum and the leaf -blade of Tropseolum): 

 when one is longer than either of the others, the body or the mem- 

 ber is cylindrical or prismatic in form when the two shorter axes 

 are equal (e.g. the stem generally), and of a flattened form when 

 one of the shorter axes is longer than the other (most leaves). 



In most cases two opposite ends are distinguishable in the body 

 or member, a base and an apex. The base is in all cases the end 

 by which the body is attached to the substratum, or the members 

 to each other, the free end being the apex. The axis or imaginary 

 line joining the base and the apex, whether or not it be longer 

 than the other axes, is termed the organic longitudinal axis. 

 When the body shows no distinction of base and apex (e.g. 

 Spirogyra), there is no organic longitudinal axis. 



Any section, real or imaginary, made parallel to the longitudinal 

 axis, is a longitudinal section : it is a radial longitudinal section 

 if it includes the longitudinal axis : it is a tangential longitudinal 

 section if it does not include it. A section made at right angles to 

 the longitudinal axis is a transverse section : the section of the 

 longitudinal axis is the organic centre of the transverse section, 

 and it commonly is also the geometrical centre of the transverse 

 section, but occasionally the geometric and organic centres do not 

 coincide. Thus, in transverse sections of tree-trunks, 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 transverse sections. 



Two kinds of symmetry may be distinguished ; the multilateral, 

 including the radial ; and the bilateral, including the isobilateral 

 and the zygomorvhic. 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. MnlHlfit,,;,l n,nl Radial Symmetry. Absolute multilateral 

 symmetry is only presented by a body or member which is 



