THE AXGIOSPERMAE ini 



we have already mentioned as being a common cause of zygomorphy, and 

 such cases are too numerous to specify. 



Reduction in the most general sense seems to have been concomitant 

 with floral evolution. Spiral flower types are usually polymerous, the cyclic 

 flowers on the other hand show a general stabilization at the level of 4 or 

 5 parts per whorl in Dicotyledons and 3 per whorl in Monocotyledons. A 

 corresponding reduction in the number of cycles takes place. Spiral flowers 

 may have 15 or 16 tiers of parts, but the majority of Dicotyledons are 

 normally pentacyclic with five whorls (calyx, corolla, stamens (2), carpels) 

 and the more advanced familes are normally tetracyclic, with only one whorl 

 of stamens. 



It is a significant fact that when the corolla is suppressed the sepals 

 often become transformed into the semblance of petals and are called 

 petaloid. The change is seen in many genera of the Ranunculaceae, i.e., 

 Caltha, Eranthis and Helleborus, where the petals are either missing or are 

 transformed into nectaries. Such an exchange of character affects other 

 organs besides sepals. Petals may become sepaloid, as in Empetrum, the 

 Crowberry, although such a modification is not very common, most flowers 

 with sepaloid perianths being only monochlamydeous, i.e., they have no 

 true corolla. 



More frequently, stamens may become petaloid, and this is the most 

 general cause of doubling. The most completely double flowers show 



Fig. 1086.— Double Rose of the " Hybrid Tea " group. (" Betty 



Uprichard ".) 



complete petalody of all the sporogenous members, both stamens and 

 carpels, with consequent sterility, but more often only the stamens, or in 

 polyandrous flowers, like Rosa (Fig. 1086) and Paeonia, only a variable 

 proportion of the stamens, are affected. In such cases intermediate organs 

 are commonly found, for example, stamens with petaloid filaments, which 



c* 



