DEVELOPMENT OF PLANTS 



433 



the typical flower of the angiosperms. It is very regular with 

 five whorls of alternating parts of five members each, i. e.: 5 

 sepals, 5 petals, 5 to 10 stamens (one or two whorls) and usually 

 5 pistils (Fig. 310, A). This type closely resembles the arrange- 

 ment of parts in the buttercups, save for the slight adhesion of 

 the calyx and receptacle (Fig. 310, D). These fleshy plants as 

 illustrated in the houseleek (Fig. 69), hen and chickens, etc., are 

 very common forms of xerophytes. Owing to their ability to 

 retain moisture (page 43) they can exist in the crevices of rocks 



Fig. 310. Simple forms of the Rosales: A, flower of Sedum, showing the 

 radial symmetry of the flower and five organs in each whorl. B, grass of 

 Parnassus (Parnassia), a member of a closely-allied family. C, flower on 

 first day of bloom, stamens converging over the pistil and encircled by row 

 of modified stamens. D, section of flower, showing slight growth of base of 

 receptacle and consequent adhesion to perianth. This flower is protandrous 

 and the stamens in shedding the spores straighten up one by one on succeed- 

 ing days and curve back towards the petals. The section shows three positions 

 assumed by the stamens. You may calculate the significance of these features. 

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