THE PKINCIPLE OF ARRANGEMENT. 45 



or ancesti^l number of carpels in an ideally complete 

 flower. 



Besides the usual alternation of whorls resulting from a 

 regular and equal displacement of every part of the whorl, 

 there may be unequal displacements ; thus, while Cistus has 

 a pentamerous flower, with strict alternation of its whorls, 

 Helianthemum has a tendency to be trimerous ; first, in the 

 two outer sepals being reduced in size, and the pistil to three 

 carpels instead of five. In this flower there are five petals, 

 but in correlation with the preceding irregularities, it will be 

 found that two pairs of petals stand superjDOsed to the sepals, 

 Nos. 3 and 5, while a single petal is over No. 4 ; Nos. 1 and 

 2, therefore, have none superposed to them. With regard to 

 the stamens, it may be added that those of Cistus consist, 

 first, of one whorl of five, the most interior and first developed 

 superposed to the sepals ; and a second whorl superposed to 

 the petals, in which the stamens are grouped into five clusters. 

 The staminal whorls arise centrifugally. 



Another cause of a change of order in the whorls results 

 from substitution of one kind for another. Thus, in the 

 female flower of Z anthoxylon, the five carpels are superposed 

 to the five sepals. In the male, five stamens now occupy 

 exactly the same place as the carpels, the corolla alternating 

 with the sepals in both kinds.* 



The interpretation I would suggest is that the sepals, 

 being the only whorl of the perianth developed, the calyx is 

 the only source for supplying the dorsal cords of the carpels 

 which thus become necessarily superposed to them. 



From what has now been said, it will be seen that the 

 arrangement of the essential organs of a flower is, as a general 



* See Figs, in Le Maout and Decaisne's Descriptive and Analytical 

 Botany, p. 324. The female flower is described as apetalous, but Payer 

 discovered rudiments of the petals. 



