ANGIOSPERMS. 



133 



the formation of the petals, and then between them five others, the distance of which 

 from the centre of the flower is however rather greater than that of the three earlier 

 ones. 



Symmetry of the Floiver. If the observations which will be found on p. i66 et seq., under 

 the head of General Morphology are now applied to the floral shoot, it is seen that true 

 symmetry and distinctly bilateral structure occur here far more commonly 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 monosymtnetrical ; 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 shape from 

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

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



Fig. 384.— Flower of HeracUum pubescens \svCs\ zygoraorphic corolla. 



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 ; 

 while in hemicvclic flowers those members at least which are arranged in whorls may 

 possibly be distributed symmetrically. If, on the contrary, the parts are all arranged m 

 whorls, they are usually distributed monosymmetrically or polysymmetrically on the 

 receptacle. Thus, for example, the diagram Fig. 375 (P- 524) can be divided symmet- 

 rically and irregularly by three planes, Fig. 376 by four, and Fig. 377 by five planes. 

 The diagrams Fig. 378 B and C, as well as Fig. 379, can, on the contrary, be symmetri- 

 cally halved by only one plane, which is at the same time the median plane. The diagram 

 Fig. 380 can be divided by the median plane into two symmetrical halves which are 



