ANGIOSPERMS. 



6ii 



than on the vegetative shoots. In contrast to the lax mode of expression used by many 

 botanists, I understand by Symmetrical Structures those which may be divided into two 

 halves, each of which is an exact reflex image of the other. If a flower can be divided 

 in this manner by only one plane, I call it simply symmetrical or monosymmetrkal ; if, on 

 the contrary, it can be symmetrically divided by two or more planes, it is, as the case 

 may be, doubly or poly symmetrical. The happy expression zygomorphic already used by 

 Braun may be applied equally to monosymmetrical flowers and to those polysymmetrical 

 ones in which the median section produces halves of quite a different form from 

 those caused by lateral section {e.g. Dicentra). I apply the term regular to a poly- 



FlG. 416.— Zygomorphic flower of Columnea Schiedeana : A entire flower after removal of two sepals ; B androecium ; 

 Cgynaeceum; Z) the coherent anthers magnified and seen from behind; £■ horizontal section of the ovary ; /=■ diagram ; 

 a anthers, n stigma, g style,y-6 ovary, d the staminode developed into a nectary,// the lateral oblique placentae. 



symmetrical flower only when the symmetrical halves produced by any one section are 

 exactly like or very similar to those produced by any other section ; or — which comes 

 to the same thing— when two, three, or more longitudinal sections divide a flower into 

 four, six, or more equal or similar portions. 



In exactly defining the symmetrical relations of a flower, the relative positions of the 

 parts, as represented by the diagram, must first of all be distinguished from the entire 

 form of the flower, such as is realised in the development of the organs. 



If attention is paid first of all only to the relative positions of the parts, it is clear 

 that they can never be distributed symmetrically in flowers with a truly spiral structure ; 



R r 2 



