DEVELOPMENT OF PLANTS 



387 



arise from the top of the structure thus formed and these organs 

 may be developed separately or there may be varying degrees of 

 mass growth as noted in the perigynous flower. 



Another feature to be noted in connection with the evolution 

 of the flower is its symmetry. In the lower types, the organs 



FIG. 267. FIG. 268. 



FIG. 267. Adhesion due to basal growth of receptacle: A, flower of saxi- 

 frage. B, section of flower, showing megasporophyll partially inclosed by 

 receptacle. C, inflorescence of red currant. D, section of flower, showing 

 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. 



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

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



of a set are alike and radially arranged about the center of the 

 flower (Fig. 267, A). This is the regular or actinomorphic type 

 of flower, meaning radially symmetrical. In many of the orders 

 of angiosperms one or more members of a set are different from 

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

 This is the irregular or zygomorphic type of flower, meaning 

 yoke-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 



