THE ANGIOSPERMAE 



1093 



mechanism. They differ in that the leguminous stamens cohere by their 

 filaments and the anthers are free, while in Compositae the opposite obtains. 

 Examples of synstemony occur in other families, e.g., Impatiens and Oxalis, 

 but it is on the whole an exceptional condition (Fig. 1060). Union of the 

 carpels is, on the contrary, a common feature of floral organization. As in 

 the other organs, it varies in extent, from union only at the base of the 

 ovaries, as in Tropaeohun, to the stage where there is complete union of 

 ovaries, the styles or at least the stylar branches remaining free, as in 

 Liniitn, or to the further stage of the fusion of styles, but with the 

 stigmas separate as in Oenothera. Finally there are innumerable cases in 

 which union of all parts of the gynoecium is so complete that their indivi- 

 duality is lost in the united structure. A very unusual state is shown by 

 Vinca and Asclepias, in which the ovaries are free but the styles and stigmas 

 are joined. 



A distinction is drawn from the developmental standpoint between 

 various degrees of carpellary cohesion. Carpels which are wholly free are 

 called apocarpous, but cases exist in which 

 carpels really free may appear to be united, 

 e.g., Nigella and Hydrocharis, the carpels 

 not being joined to each other but to a 

 common axis of receptacle tissue. Carpels 

 which are united to each other to form a 

 compound structure are called by the general 

 term coenocarpous (Fig. 1061). Where 

 the ovary shows septation into distinct 

 loculi, i.e., internal spaces, corresponding 

 in number to the united carpels, the condi- 

 tion is called syncarpous, and where there 

 is no septation and only a single loculus, 

 common to all the carpels, it is called 

 paracarpous. The latter condition has been 

 attributed to a shift of the fertile region, i.e., 

 the ovule-bearing placentae, into the upper 

 portion formed by the united styles, and 

 all stages may be observed between paracar- 

 pous gynoecia with a distinctly syncarpous 

 base, to those in which the latter has been 

 suppressed. The paracarpous gynoecium 

 naturally has rarely any styles, though the 



stigmas may remain free. The coenocarpous gynoecium may thus be re- 

 garded as having three zones: syncarpous base, paracarpous styles and 

 apocarpous stigmas, which may be all present, in varying degrees of 

 development, but any of which may be suppressed. Apocarpous gynoecia 

 are characteristic of flowers with large or elongated receptacles while the 

 close approximation required for union of the carpels can only occur in 

 flowers with contracted receptacles. 



I 



Fig. 



B C D 



1 06 1. — Troll's interpretation 

 of carpellary cohesion. A, B, 

 Typical coenocarpous gynoe- 

 cium, showing, from below, 

 syncarpous, paracarpous and 

 apocarpous regions, with the 

 corresponding transverse sec- 

 tions. C, Typical syncarpous 

 gynoecium. D, Syncarpous 

 gynoecium of Colchiciim, with 

 long, free stylodia, which are 

 really free, elongated stigmas, 

 the paracarpous stylar region 

 being almost suppressed. 

 {After Troll.) 



