454 



PART IV. CLASSIFICATION. 



The symmetry may be isobilateral ; in this case the flower is 

 divisible into symmetrical halves in two planes, but the halves 

 produced by section in one plane are unlike those produced by 

 section in the other plane. Thus, a regular eucyclic dimerous 

 flower (e.g. Circcea lutetiana, Fig. 270 B ; Fraxinus dipctala\ 

 is symmetrically divisible in the median and lateral planes, but the 

 halves produced by the median section differ from those produced 

 ~by the lateral section. This is true also of some regular hetero- 

 cyclic flowers, such as those of the Cruciferas, Jasminum, Olea euro- 

 pcea, Cornus, Hamamelis, the whorls of which are 2- or 4-merous, 

 and of the somewhat peculiar flower of Dicentra. 



The symmetry may be zygomorpMc, that is, the flower may be 

 monosymmetrical, there being only one plane in which it is sym- 

 metrically divisible. Monosymmetry is characteristic of irregular 



flowers, whether eucyclic 

 or heterocyclic ; of flow- 

 ers, that is, in which the 

 members of one or more 

 whorls differ in various 

 respects among themselves, 

 accompanied frequently by 

 a reduction in the typical 

 number of members in one 

 or other of the whorls, fre- 

 quently of the androacium : 

 it is, in fact, to irregular 

 flowers that the term 

 zygomorphic is specially 



applied in Descriptive Botany. Such a flower usually presents a 

 clear distinction into two diverse portions, an anterior and a 

 posterior, separated by the lateral plane, whilst the two lateral 

 halves about the median plane are symmetrical ; hence it is clearly 

 dorsiventral (Fig. 271). 



Dorsiventrality is presented by some flowers which, so far as 

 their early development is concerned, or even so far as is shown by 

 their floral diagram, are actinomorphic, isobilateral, or simply zy- 

 gomorphic, the dorsiventrality being due to the subsequent irregu- 

 lar development of some of the floral leaves ; as in some eucyclic 

 flowers (e.g. among Monocotyledons, Amaryllis, Gladiolus ; among 

 Dicotyledons, Dictamnus, and other Rutese, species of Impatiens, 

 Pelargonium), and in some heterocyclic flowers (e.g. some Scrophu- 



FIG. 270 X Diagram of the tetramerons flower 

 of Fuchsia, showing the four planes of symmetry. 

 B Diagram of the dimerous flower of Circsea, show- 

 ing isobilateral symmetry. 



