CYCLIC FLOWERS 



71 



XiX. 



and similar halves by vertical cuts made in several different 

 planes (directions); whereas in the other two plants named 

 me parts are not regular, and the flower can be equally 

 halved by a cut made only in one direction — i.e. passing 

 through the middle of 

 the standard and be- 

 tween the two keel- 

 petals. All these 

 flowers mentioned are 

 said to be symmetrical, 

 because it is possible 

 to divide them into 

 two similar halves. The 

 Geranium and Hya- 

 cinth flowers are sym- 

 metrical in several 

 planes (directions), or 

 actinomorphic : the Pea 

 and Clover flowers 

 are symmetrical in 

 one plane (direction) 

 only, or zygomorphic. The zygomorphy of flowers is caused 

 by the irregular growth, by suppression, by fusion, or by 

 doubling of their parts.* When a cyclic flower cannot be 

 divided into two equal halves, it is said to be asymmetrical 

 {e.g. some members of the Pink-family). It is convenient 

 to have some method of describing the relative positions 

 occupied by the parts of a flower and the stem on which 

 the latter is inserted. The half of the flower which faces 

 the bract, or leaf, in the axil . of which the flower stands, is 

 said to be the anterior h'alf; whilst the half of the flower 

 which faces the inflorescence - axis is the posterior half. 

 The plane dividing the flower vertically into its posterior 

 and anterior halves is the transverse plane. Whereas 



-.as 



.hr. 



Fig. 100. — Floral diagram of same flower. 



* In the majority of books a flower is not said to be zygomorphic when 

 its solitary deviation from an actinomorphic flower is tlae suppression of 

 some of its carpels. For example, many regular flowers possess five 

 sepals, five petals, but only two carpels. If we assume that each floral 

 leaf cut must be exactly halved, these flowers are, strictly speaking, 

 symmetrical only in one . plane : nevertheless, they are usually described 

 as actinomorphic. 



