the perianth is composed of a varying number of petals and sepals, 

 which are not always clearly separable, as in the Water-lilies. The 

 latter recall in many ways the Helobieae, and it is not impossible 

 that they are really related to them. 



The single terminal ovule of the lower Centrospermse is replaced 

 in the higher forms by an axial placenta, upon which are borne 

 numerous ovules. Some of them have several carpels which may be 

 united into a several-chambered ovary (e.g. Mesembryanthemum). 

 The more specialized types, like Dianthus, Silene, Portulaca, etc., 



FIG. 406. Asarum Canadense. 



have showy petals, and the calyx is often tubular. In Mesembry- 

 anthemum (Fig. 409) many of the numerous stamens are transformed 

 into petaloid staminodia. 



In the higher Choripetalae the calyx is usually composed of united 

 sepals, and the parts of the flower are constant in number. The sta- 

 mens, however (Myrtacese, Rosacese), may be more numerous, and 

 sometimes the carpels also, suggesting an affinity with the Ranales. 

 With the exception of some of the Rosiflorae, also, the carpels are 

 almost always united into a compound pistil. 



Among the less specialized forms the flowers are radially sym- 



