16 



MORPHOLOGY OF ANGIOSPERMS 



in two categories, and three have been proposed, as follows: (1) 

 actinomorphic, in which the planes of symmetry are as numer- 

 ous as the members of a cycle; (2) isobilateral, in which there 

 are two planes of symmetry, but the halves produced by one 

 plane are unlike those produced by the other (Dicentra, Cru- 

 ciferae, etc.) : and (3) zygomorphic, in which there is only one 

 plane of symmetry (Fig. 2). These categories are expressions 

 of certain laws of growth, and that they are somewhat funda- 



c 



Fig. 2. — A, radial symmetry (Lilium tigrinum); B, isobilateral symmetry (Capsella 

 Bursa-pastoris) \ C, zygomorphic symmetry (Scrophularia nodosa). 



mental may be inferred from the fact that they are persistent 

 through great groups of plants. 



While these and other evolutionary tendencies are to be 

 observed among flowers, it is evident that they are not neces- 

 sarily expressed simultaneously. For example, the spiral and 

 cyclic arrangements are associated in Ranunculus, zygomorphy 

 is associated with polypetaly and hypogyny among the papil- 

 ionaceous Leguminosae, epi ryjiy is associated with polypetaly 

 among the Umbelliferae, and sympetaly and zygomorphy are 

 associated with hypogyny among the Labiatae. It is among 

 the Compositae that practically every evolutionary tendency 

 mentioned finds its highest expression. Tt is only by striking 

 an average that such characters may be used in roughly placing 

 a family in its evolutionary position, commonly called its 

 " relative rank." 



The classic memoir on the organogeny of the flower is 

 Payer's Traite d'organogenie de la fleur (1857), but the sub- 

 ject has not been developed since as it deserves. In the case 

 of spiral flowers, in which the torus elongates more or less, the 



