31 THE STRUCTURE OF FLOWERS. 



portion of plants in this class have alternate leaves and 

 5-nierous flowers, this correlation alone would be almost 

 sufficient to prove that the latter issued out of the com- 

 monest or f type of phyllotaxis. But since the sepals are 

 sometimes decidedly quincuncial, as are those of Digitalisj 

 and the petals frequently so, we have undoubted proof that 

 they represent cycles of this angular divergence. 



As with other numbers, fives may arise by symmetrical 

 increase from fours, or decrease from sixes ; though in by 

 far the greater number of instances it is a primitive number, 

 as stated above. As a rare instance of symmetrical decrease 

 may be mentioned Lytlirum Salicaria, which has usually the 

 central floret of each axillary cyme 6-merous, but the lateral 

 ones only 5-merous. As an instance of five parts to a whorl 

 amongst Monocotyledons, may be mentioned the stamens of 

 Strelitzia regina ; but this number is obviously due to the 

 suppression of a stamen. 



Although whorls of fives are cycles of the f divergence, 

 and usually follo^\L after an alternate arrangement in the 

 foliage, yet it is quite possible to change abruptly from 

 opposite leaves or bracts to whorls of fives in the flower, as 

 may be seen in Hypericum and Dianthus. This arrangement, 

 as I have elsewhere shown, is that most easily acquired 

 when opposite and decussate leaves become alternate by the 

 development of internodes (see pp. 11 and 18). 



Hexamerous Whorls. — A floral whorl of six parts is, in 

 most cases, as amongst Monocotyledons, the result of the 

 combination of two whorls of three each — as the androeciuni 

 of Berberis, Tulip, or perianth of the Lily of the Yalley. It 

 may, however, arise from symmetrical increase, as, for 

 example, in the orders Meliacece and OlacinecB. In the 

 former, there are 18 genera with alternate leaves and 

 5-merous flowers ; 9 with 4-5-merous ; 4 with 4-merous ; 



