354 



EVOLUTION OF THE FLOWER 



the others, thus destroying the radial symmetry (Fig. 268). 

 This is the irregular or zygomorphic type of flower, meaning 

 yolk-form. Such flowers can be cut into two similar halves in 

 but one plane. 



It must not be understood that evolution of the flower has 

 progressed steadily through the various changes outlined above 



Fig. 267. 



Fig. 268. 



Fig. 267. Adhesion due to basal growth of receptacle : A, flower of 

 saxifrage. B, section of flower, showing megasporophyll partially inclosed 

 by receptacle. C, inflorescence of red currant. D, section of flower, show- 

 ing the receptacle forming a sporangial cavity that is covered at the top 

 by the megasporophylls. The other organs arise from the top of this 

 structure, the microsporophylls adhering to the corolla. 



FlG. 268. Irregular or zygomorphic flower of honeysuckle: ca, calyx; 

 c, corolla of five unequal cohering petals ; s, stigma. 



and that consequently in the following lessons we can begin with 

 the most primitive type and proceed by regular steps to the 

 highest forms. The various orders of angiospcrms have doubt- 

 less been derived from several distinct stocks and they have 

 not only varied in different degrees but especially will it be noted 

 that some orders have a tendency to emphasize certain forms of 

 these variations, while in other alliances, the variations will 



