GROUP IV. PHANEROGAMIA : ANGIOSPERM.E. 507 



in Dicotyledons when, as is frequently the case, the prophylla 

 are not exactly lateral, but converge anteriorly or posteriorly, or 

 are even anterior and posterior (Fig. 320 JB). 



When the one or the two prophylla are suppressed, the position 

 of the sepals may be the same as if the prophylla were present 

 {e.g. Cruciferae) ; but, more commonly, the first sepals are de- 

 veloped in the places of the missing prophylla. Thus, in the 

 absence of two lateral prophylla, the first two sepals of a tetra- 

 merous calyx are lateral (e.g. tetramerous calyx of Francoa, 

 Epilobium, Clarkia) ; again, in a trimerous calyx, .the first nor- 

 mally anterior sepal tends to be posterior when the single pos- 

 terior prophyllum is absent (e.g. Orchis, Musa) ; and, further, in a 

 pentamerous calyx the first and second sepals are postero-lateral 

 (e.g. Primula, Reseda) since the first sepal occupies the place of 

 prophyllum a, and the second that of prophyllum /?, with slight 

 posterior convergence. 



The Symmetry of the Flower. The flower presents all the 

 varieties of symmetry which are discussed in Part I. (p. 8) ; 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 maybe 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. 9). The number of these planes of 

 symmetry depends upon the numerical constitution of the flower. 

 Thus a regular eucyclic trimerous flower (e.g. Lilium and other 

 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. 322 B). Similarly, the pentamerous flower 

 of Primula, Geranium, species of Campanula, is divisible in five 

 planes (Fig. 322 A}. But where the flower is tetramerous (e.g. 

 Fuchsia, Rhamnus catliartica, Euonymus 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. 323 ^4) ; or, 

 again, where the flower is hexamerous (e.g. species of Sedum) it 

 is symmetrically divisible in twelve planes, but the halves produced 



