106 MORPHOLOGY OF THE ANGIOSPERMS 



less common than to the corolla — Proteaceae. Fusion to the carpels, 

 where all the outer organs are together fused to the gynoecium in 

 perigyny and epigyny, is common. Fusion to carpels alone is rare — 

 Sarcandra, Monimiaceae. Fusion to styles and stigma, with the forma- 

 tion of a gijnostemium, is characteristic of the Orchidaceae and Stylid- 

 iaceae. 



Adnation may be by filaments for part or for all their length, where 

 the anthers are sessile on other organs. (The term sessile is unfortunately 

 applied both to anthers where free stamens consist of anthers only, and 

 to those where filaments merge with the uniting organ.) Adnation of 

 the anther to other organs varies in extent, from attachment near the 

 base only to union by the entire dorsal or ventral surface — the dorsal 

 surface to a sepal in many Proteaceae, the entire ventral surface to the 

 gynoecium in the orchids. In Yiscum, fusion of the entire stamen to the 

 sepal is intimate, and all external evidence of the fusion is lost; the 

 sporangia are apparently borne on the ventral surface of the sepal, 

 which is described as "polliniferous." 



Stages in fusion — ontogenetic or phylogenetic — of stamen to adjacent 

 organs can be seen in many taxa. External evidence of the fusion may 

 remain, the filament forming a ridge on the surface of the petal or sepal, 

 or the only evidence may be internal, in the presence of the vascular 

 bundles of the two organs, which lie, radially, side by side. Fusion, 

 when phylogenetically established, may include the vascular tissues, and 

 the bundles of the two organs may be merged and histologically in- 

 distinguishable. 



The Androecium under Zygomorphy 



Number, form, and structure of stamens are greatly modified in the 

 development of zygomorphy. Some of the stamens become sterile, are 

 reduced in size, and lost, leaving little or no evidence in external form, 

 though sometimes internal vascular traces may persist. Often, where 

 there is more than one whorl of stamens, one entire whorl, or some 

 members of either, or of both, whorls are suppressed. Of a whorl of 

 six in Carina, one remains fertile, four become petaloid staminodia, and 

 one is lost; in the Bignoniaceae, two stamens are fertile and three sterile. 

 Correlated with sterilization and loss in the androecium under develop- 

 ment of zygomorphy, are modifications in size and form, in time of 

 maturation, and in adnation of the stamens to other organs. A number 

 of terms are used in descriptive taxonomy for these variations but they 

 have li\.tle morphological significance — for example, didtjnomous, where 

 the androecium consists of two pairs of stamens of unequal length; 

 tetradynamous, of four long and two short stamens. 



