DEVELOPMENT OF PLANTS 



369 



variations that appeared in the evolution of the flower is asso- 

 ciated with the shortening of this receptacle. This is brought 

 about by the checking of the apical growth of the receptacle and 

 is often associated with a more or less extended growth of its 

 basal region. These changes affected the flower in a most pro- 

 found way. A crowding resulted, organs were reduced in number 

 and their spiral arrangement upon the receptacle 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 angiosperms. As the cyclic habit 

 became established, so the number of organs in each whorl be- 



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

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

 tacle forming a cup about the megasporophylls, nig, and bearing the other 

 organs of the flower. C, inflorescence of corafrey, Symphytum. D, flower 

 enlarged in section to show adhesion of microsporophylls, mi, to the tubular 

 corolla. 



came constant. Thus at a certain point in the evolution of the 

 monocotyledons you will find the organs usually in threes, where- 

 as in cyclic dicotyledons whorls with four or five organs are the 

 rule. 



The crowding of the organs on the receptacle also caused them 

 25 



