DEVELOPMENT OF PLANTS 351 



is said to be complete, consisting of all the organs that are 

 normally associated in the flower. The receptacle or region upon 

 which the sporophylls and perianth are arranged, is often some- 

 what elongated in the simpler types of flowers, thus giving room 

 for the separate attachment of the organs. In such cases the 

 various parts are spirally arranged and very numerous (Fig. 

 264). This spiral type of flower is characteristic of a large 

 number of the lower orders of angiosperms and, as stated above, 

 is suggestive of a strobilus. 



The shortening of the receptacle is one of the most important 

 variations that appear in the evolution of the flower. This is 

 brought about by the checking of the apical growth of the re- 

 ceptacle and is often associated with a more or less extended 

 growth of its basal region. These changes affected the flower 



Fig. 264. 



Fig. 265. 



Fig. 264. Flower of strawberry with elongated receptacle bearing nu- 

 merous spirally arranged sporophylls : A, open flower. B, flower in sec- 

 tion, showing arrangement of parts upon receptacle, r, which forms a 

 shallow cup at base bearing the perianth and microsporophylls. C, the 

 fruit or enlarged receptacle bearing the minute spirally-arranged mega- 

 sporophylls. 



Fig. 265. Flowers with shortened receptacles : A, flower of Pyrola with 

 calyx and corolla arranged in whorls or cycles. B, section of flower, show- 

 ing all the organs in cycles. C, flower of geranium. D, flower with cor- 

 olla removed to show the coherence of the five megasporophylls that re- 

 sults from the shortening of the receptacle. 



