MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 131 



a characteristic shape consequent on that relative position 

 towards external and internal forces, which the mode of 

 growth entails. Every member of the aggregate presents 

 itself in a more or less peculiar way towards the light, towards 

 the air, and towards its point of support; and according to 

 the relative homogeneity or heterogeneity in the incidence of 

 the agencies thus brought to bear on it, will be the relative 

 homogeneity or heterogeneity of its shape. 



216. Before passing from this a priori view of the 

 morphological differentiations which necessarily accompany 

 morphological integrations, to an a posteriori view of them, it 

 seems needful to specify the meanings of certain descriptive 

 terms we shall have to employ. 



Taking for our broadest division among forms, the regular 

 and the irregular, we may divide the latter into those which 

 are wholly irregular and those which, being but partially 

 irregular, suggest some regular form to which they approach. 

 By slightly straining the difference between them, two cur- 

 rent words may be conveniently used to describe these sub- 

 divisions. The entirely irregular forms we may class as 

 asymmetrical literally as forms without any equalities of 

 dimensions. The forms which approximate towards regu- 

 larity without reaching it, we may distinguish as unsymmetri- 

 cal: a word which, though it asserts inequality of dimensions, 

 has been associated by use rather with such slight inequality 

 as constitutes an observable departure from equality. 



Of the regular forms there are several classes, differing in 

 the number of directions in which equality of dimensions is 

 repeated. Hence results the need for names by which sym- 

 metry of several kinds may be expressed. 



The most regular of figures is the sphere : its dimensions 

 are the same from centre to surface in all directions; and if 

 cut by any plane through the centre, the separated parts are 

 equal and similar. This is a kind of symmetry which stands 

 alone, and will be hereafter spoken of as spherical symmetry.. 



