186 THE STRUCTURE OF FLOWERS. 



successively. If the stamens be very numerous they 

 usually emerge in centripetal order, as in Buttercups ; but 

 they may form "centrifugal groups," as in Hypericum ; the 

 numerous stamens of Gistus and Helianthemum, as well as 

 of Cactus, Opuntia, and Mesemhryanthemum, and the Loasece, 

 are also centrifugal in their development. Lastly, if the 

 carpels form a whorl, they, too, emerge simultaneously ; but 

 if they be numerous and spirally arranged they emerge and 

 develop in succession. 



There are some additional points to be observed. The 



first is the method of change from tetramerous to pen- 



/ .? 



tamerous in the same plant. Thus in Celastrus scandens, 

 if the flower be tetramerous, the sepals appear in pairs, the 

 antero-posterior first, then the lateral pair afterwards. If 

 the flower be pentamerous the sepals arise in succession 

 quincuncially, the numbers 1 and 3 being anterior ; numbers 

 4 and 5 are lateral, and number 2 posterior. 



Now, by referring to the diagrams above, it will be 

 seen that this order is in exact agreement with the usual 

 method of passing from opposite to alternate arrangements 

 in the foliage. The correct angular distance or divergence 

 being acquired immediately in the case of the calyx, by 

 shifting the position of the parts so that the divergence of 

 144° is obtained. In the case of foliage, this is only secured 

 after several internodes (see p. 18). 



