THE ANGIOSPERMAE 



1095 



flowers, where they are normally aligned with the sutures between the 

 coherent petals. 



In many Dicotyledons the androecium consists of two whorls of stamens, 

 whose relative positions illustrate the principle of alternation. The stamens 

 of the outer whorl alternate with the petals and are therefore opposite the 

 sepals, i.e., they are antisepalous. Those of the inner whorl are opposite 

 the petals and are called antipetalous. This twofold alignment is called 

 diplostemony. 



Alternation of the parts in successive floral whorls is so fundamental in 

 the structure of all cyclic types of flower that an exception to the rule calls 

 for some explanation. Such an exception is obdiplostemony, which implies 

 that the stamens of a single whorl, or those of the outer whorl if there be 

 more than one, stand opposite the petals, i.e., they are antipetalous instead 

 of, as usual, antisepalous (Fig. 1063). This condition may be seen, for 

 example, in Primula, Vitis, Geranium, Silene and Viscaria and other Caryo- 

 phyllaceae and in the Chenopodiaceae, Ericaceae, ^ 



Oxalidaceae, etc. Several suggestions have been 

 made towards bringing this anomaly into con- 

 formity with the general rule. The simplest is 

 that an outer whorl of stamens, occupying 

 the normal antisepalous position, has been sup- 

 pressed. Evidence for this is the presence in 

 the receptacle of Geranium maculatum of a ring 

 of vestigial trace-bundles outside the two extant 

 stamen whorls and alternating with the sepals, 

 as would be expected if they mark the position 

 of the vanished stamens. The value of such 

 anatomical evidence has often been doubted 

 (see the next section) but this is a case in which 

 it is diflicult to reject it. So far as Primulaceae 

 is concerned the hypothesis of a suppressed whorl of stamens is made prac- 

 tically certain by the presence of a whorl of sterile stamen rudiments in 

 the antisepalous position in Samohis and Soldanella, both members of 

 that family (Figs. 1064, and 1065). 



Another suggestion is that the petal and its superposed stamen are a 

 single morphological unit, the petals in such cases being the product of the 

 abaxial fusion of a pair of staminal stipules. Lateral outgrowths of the 

 stamen filament frequently occur, whether they are truly stipular or not, 

 and that these may fuse behind the stamen seems to be borne out by the 

 formation of the " corona " or inner corolla in the flowers of some Amaryl- 

 lidaceae, e.g.. Narcissus. The bifid outline of the petals in many Caryophyll- 

 aceae, especially well marked in Stellaria, accords with this idea and it may 

 be the explanation in some cases, if not in all. A third theory, due to Payer 

 and involving less supposition than the others, is that there has been an 

 apparent inversion of the stamen whorls in the flower, due to the larger size 

 and more rapid growth of the primordia of the antisepalous stamens and of 



Fk;. 1063. — Floral diagram of 

 Primula. Stamens of the 

 single whorl opposite the 

 petals. 



