DEVELOPMENT OF PLANTS 



385 



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 and no one character is of 



. >- ^•^' .'M fij 



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, 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. 



more value in enabling us to state whether a plant is of high or 

 low rank. As the cyclic habit became established, so the number 

 of organs in each whorl became constant. Thus at a certain 

 point in the evolution of the monocotyledons you will find the 

 organs usually in threes, whereas in cyclic dicotyledons whorls 

 with four or five organs are the rule. 



The crowding of the organs on the receptacle also caused them 

 to develop en masse (as a unit), not as separate organs. This 



