352 



EVOLUTION OF THE FLOWER 



in a most profound way. A crowding resulted, organs were 

 reduced in number and their spiral arrangement upon the re- 

 ceptacle became so flattened that the various sets of organs 

 appear to arise in whorls or cycles (Fig. 265, A, B). These 

 cyclic flowers are characteristic of all the higher orders of angio- 

 sperms. The crowding of the organs on the receptacle also 

 caused them early in their development to come in contact with 

 one another laterally and to cohere, forming a united structure. 

 This tendency is especially noticeable in the megasporophylls, 



D 



FIG. 266. Forms of adhesion that result from shortening of receptacle: 

 A, flower of rose. B, section of flower, showing the lower portion of 

 receptacle forming a cup about the megasporophylls, mg, and bearing the 

 other organs of the flower. C, inflorescence of comfrey, Symphytum. D, 

 flower enlarged in section to show adhesion of microsporophylls, mi, to 

 the tubular corolla. 



which very frequently cohere into a compound megasporophyll 

 (Fig. 265, C, D}, and in the same way the sepals and petals 

 become united, forming a more or less tubular calyx and corolla 

 (Fig. 266, C). The crowding also led to the adhesion of the 

 organs of adjacent sets or whorls. This is often seen in the 

 case of the petals and microsporophylls, the latter organs appear- 

 ing to arise from the corolla owing to the union of the lower 

 portion of the filament with the base of the petals (Fig. 266, D). 



