244 MORPHOLOGY OF THE ANGIOSPERMS 



— with an external appearance of one — the stigma of only one of the 

 component carpels is usually present; the stigmas of the other carpels 

 are abortive or lost. This is the common condition in pseudomonomerous 

 gynoecia. Rarely, in these taxa, the stigma is borne on a sterile carpel 

 over a placenta common to a sterile and the fertile carpels. In many 

 taxa, number of stigmas or stigmatic lobes is not correlated with 

 carpellary number; evidence from ontogeny and vascular anatomy and 

 from the structure of the ovary and that of related taxa is essential in 

 determining carpel number. 



In syncarpous gynoecia, the stigmas commonly stand over the carpel 

 midrib; they are carinal. But, in some taxa, notably those with parietal 

 placentation — Resedaceae, Droseraceae — they stand over the "commis- 

 sures," lines of fusion of carpel margins; these stigmas are commissural. 

 This unusual position has long been correctly interpreted by taxonomists 

 and morphologists as the result of the fusion of the lobes of a divided 

 stigma with those of the adjacent carpels. Under the theory of carpel 

 polymorphism, the commissural stigma is considered highly complex 

 in carpellary nature; it consists of halves of two "solid" carpels, fused 

 laterally with a "valve" carpel, forming a compound structure made up 

 of one-half plus one plus one-half carpels. 



The commissural position of the stigma is shown by ontogeny and 

 vascular anatomy to be the result of the union of carpels, margin to 

 margin; the lobes of the forked stigmatic tip of the carpel are fused 

 laterally with the lobes of the adjacent carpels. The double stigmas so 

 formed, each consisting of halves of the placentae of different carpels, 

 stand alternately with the carpellary midribs. The vascular bundles 

 that supply the stigma belong to different carpels — they are the ventral 

 bundles of adjacent carpels. The midrib vascular bundles lead to the 

 sinuses between the stigmas. The general form of the ovary top, in 

 many taxa with parietal placentation, is misleading in its suggestion of 

 carpellary position. 



Syncarpy with Adnation to the Receptacle. The morphology of the 

 syncarpous gynoecium is frequently complicated by adnation — inter- 

 nally, by fusion of carpels laterally or obliquely with the receptacle; 

 externally, by fusion with the outer floral organs — perigyny and epigyny. 

 In less specialized families, the receptacle is frequently prolonged be- 

 yond the bases of the uppermost carpels and may be adnate to the 

 carpels. The extent of this adnation varies with the form of the re- 

 ceptacle and carpel base, and with the angle at which the carpels stand 

 If the adnate carpels are open, tissues of the receptacle may close the 

 opening proximally. A line of demarcation between tissues of carpel 

 and receptacle is difficult or impossible to determine. 



In studies of flower structure, delimitation of the receptacle is often 



