508 



PART III. — THE CLASSIFICATION OF PLANTS. 



A B 



Fig. 322. — A Diagram of the pentamerous flower 

 of Primala, showing the five planes of symmetry ; 

 the stamens are antipetalous ; there are no pro- 

 phylla. B Diagram of the trimerous flower of 

 Lilium, showing the three planes of symmetry. 

 (After Eichler.) 



by the section in six of the planes are unlike those produced by- 

 section in the other six planes. 



The symmetry may be isohilateral ; 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 pro- 

 duced by section in the 

 other plane. Thus, a re- 

 gular eucyclic dimerous 

 flower (e.g. Girccea luteti- 

 ana, Fig. 323 B ; Fraxinus 

 dipetala), 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 

 heterocyclic flow^ers, such as those of the Gruciferae, Jasminum, 

 Olea europcEa, Cornus, Hamamelis, the whorls of which are 2- 

 or 4- merous, and of the somewhat peculiar flower of Dicentra. 



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

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

 metricallv divisible. This condition is to be found in regular 



heterocyclic flowers, and 

 is the result of oligomery 

 of the whorls, generally of 

 the gyneeceum, rarely of 

 the androecium. Flowers 

 of this type are common 

 among Dicotyledons (e.g. 

 in the Ribesiacese, Apo- 

 cynaceae, Boraginaceae, 

 Solanacea3, Gentianaceae, 

 Gampanulaceae, Gompo- 

 sitse, Rosaceae, Saxifrag- 

 aceae, Umbelliferae, etc.), 



Fig. 323. — A Diagram of the tetramerous flower 

 of Fuchsia, showing the four planes of symmetry. 

 JB Diagram of the dimerous flower of Circaea, show- 

 ing iaobilateral symmetry. 



the oligomerous gynaeceum having 1-4, generally 2, carpels, the 

 rest of the flower being pentamerous or hexamerous. In this case 

 the plane of symmetry is determined by the position of the carpels. 



