GROUP V. ANGIOSPERM.E. 



453 



Dicotyledons) is not the result of suppression, but is itself typical : 

 in other words, these flowers are probably to be regarded, not as 

 reduced, but as primitive, belonging to plailts which are, it may 

 be, of a relatively low type among Phanerogams, but which are on 

 the up-grade^ and not on the down-grade of organisation. 



The Symmetry of the Flower. The flower presents all the 

 varieties of symmetry which are discussed in Part I. (p. 4) ; these 

 are mainly determined by the number and the relative develop- 

 ment of the floral leaves, and in a few cases by the development 

 of the floral axis or receptacle. 



The symmetry may be radial or actinomorphic. When an 

 eucyclic flower is also regular, that is, when the members of each 

 whorl are similar to each other in size and form, it can be divided 

 into symmetrical halves 

 by sections made in two 

 or more planes, the halves 

 produced by section in one 

 plane being similar to 

 those produced by section 

 in one or more other 

 planes. Such a flower is 

 poly symmetrical (see p. 

 6). The number of these 

 planes of symmetry de- 

 pends upon the numerical 

 constitution of the flower. 

 Thus a regular eucyclic 

 trimerous flower (e.g. 

 many Monocotyledons) can 

 be so divided in three planes, the median and the two diagonals, 

 that all the three pairs of resulting halves are exactly alike (Fig. 

 269 B). Similarly, the pentamerous flower of Primula, Geranium, 

 species of Campanula, is divisible in five planes (Fig. 269 A}- 

 But where the flower is tetramerous (e.g. Fuchsia, Euonynni* 

 europceus), there are but two planes of section, the median and the 

 lateral, which will give exactly similar halves, though the flower 

 is also symmetrically but diversely divisible in the diagonal planes 

 (Fig. 270 A); or, again, where the flower is hexamerous (e.g. 

 species of SeduuVt it is symmetrically divisible in twelve planes, 

 but the halves produced by the section in six of the planes are 

 unlike those produced by section in the other six planes. 



A B 



FIG. 239. A Diagram of the pcntameroua flower 

 of Primula, showing the five planes of symmetry; 

 the stamens are antipetalons ; there are no pro- 

 phylla. B Diagram of the trimerons flower of 

 Lilium, showing the three planes of symmetry. 

 (After Eichler.) 



