COHESION AND SUPPRESSION. 



261 



one is driven to the conclusion that it has been one of 

 the main factors in the evolution of the flower.* 



The crux always arises over the transitional cases, 

 e. g. in the Cruciferae that of the four stamens of the 

 inner whorl which are grouped in pairs ; the ontogeny 

 shows that at the earliest stage the pair of stamens 

 is represented by a single one, which only at a later 

 stage branches into two. Eichler, guided by these 

 facts of ontogeny, and by the fact that occasionally a 

 single stamen replaces each pair, holds that the 

 2-merous condition of both whorls was the original 

 one, and that the pairs of the inner whorl have' arisen 

 later by positive dedoublement. 



Celakovsky's view is, on the contrary, that the 

 approximation of the stamens in pairs is due to nega- 

 tive dedoublement, that what has occurred already in the 

 outer whorl is at present occurring in the inner whorl, 

 viz. a progression towards dimery of the androecium. 



Velenovsky holds that the Rhceadales are founded 

 on a dimerous type derived from the immediate 

 ancestry. This is proved, he thinks, by such a fact 

 as the 4-merous androecium observed in Papaver by 

 Warming and Battandier, that of the normal flower, 

 as in the Capparidacese, being a more recent reversion 

 to the pleiomerous condition of the still remoter 

 ancestry. But these poppies were growing under 

 specially impoverished conditions which would act as 

 a further stimulus to the already present tendency in 

 the order Papaveracea3 for reduction in the various 

 whorls. Velenovsky admits the remote ancestral 

 nature of the normal pleiomerous androecium, but it 



* Wernham (" Floral Evolution ; with particular reference to the 

 Sympetalous Dicotyledons/' ' New Phytologist/ vol. x, April, 1911) brings 

 out the interesting fact that " whereas the flowers of barely 20 per cent, of 

 the total number of species of Archichlamydese have stamens equal in number 

 to, or less than, the corolla segments, an isomerous or oligomerous 

 androecium characterizes nearly 95 per cent, of the species of Sympetalse. 

 Only about 18 per cent., again, of the Archichlamydese have a pistil composed 

 of two syncarpous carpels or, in some rare cases, of one only ; in the 

 Sympetalse, on the other hand, fully 75 per cent, of the species have flowers 

 with a bicarpellary gynseceum." The Sympetalse are universally regarded 

 as a more advanced group than the Polypetalse. 



