PEELIMINART EEMARKS. 3 



these, mingling together, form a magma, soon charged with poUen-tuhes, all pervading 

 the central space and confined within it : this magma now flowing over the stigmata, the 

 poUen-tuhes penetrate their papillse, and pass down the stigmatic channels of the style, 

 eventually impinging upon the ovules; and thus the work of fertilization is effected 

 without the aid, or even the possibility of insect agency. This mechanism, tending 

 to the exact apposition of the several parts towards one another, and their appliance to 

 functional purposes, appears so wonderful, and so very like the most ingenious design, 

 that it cannot fail to excite our admiration. 



There is a considerable degree of analogy in the development and fertilization of the 

 Apbcynacece and Asclepiadacece, more so than is generally supposed. In the latter 

 family the prevailing floral structure is different. The coroUa is much smaller, its tube 

 very short, its segments equilateral and expanded : within these segments is a corona of 

 five, ten, or fifteen petaloid lobes, in one, two, or three series ; but their use is not manifest. 

 The five conspicuous stamens, alternate with the border segments of the corolla, are 

 peculiar in their construction : they are fixed to the short tube of the corolla by broad 

 filaments united into a short funnel, each continued into a free broad connective, and 

 terminated by an expanded membrane, all connivent in the centre over the stigma, thus 

 forming what may be called a stegiwm. Each connective bears inside two adnate anther- 

 cells, sometimes divided into four ; each cell consists of two distinct laminae, the outer 

 one {eocothecium) being part of the soft connective, the inner one (endothecium) being 

 separable, forming a polliniferous lobule of an oblong form, terminating at one end in a 

 thread. Here we find a singular arrangement : the thread of one of the lobules of one 

 anther is conjoined at its apex with the thread pf one of the lobules of the adjacent 

 anther, and geminately united to a small corpuscle {retinaculum) which is agglutinated 

 to one of the angles of the clavuncle, leaving them generally pendulous, but sonietimes 

 erect. Each endothecium is thick in substance, and is pitted all over with numerous 

 minute hexagonal recesses, severally containing a granule of pollen, according to the 

 analyses of Bauer and Brown*. The style is short, and surmounted in most of the genera 

 by a very large, flattened, peltate, pentagonal clavtmcle, its angles being furnished with 

 nectarial glands, somewhat as in the Apocynacece, exhibiting in the centre, at the 

 summit, two distinctly developed stigmata, often very short, which are studded all over 

 with the usual papillge. The nectarial juice of the glands is now conveyed by the agency 

 of capillary attraction along the threads of the lobules, and is diffused over their poUini- 

 ferous surface ; and under this stimulus the pollen granules expand into long tubes or 

 boyaux, which spread in all directions, a sufQ.cient number reaching and penetrating the 

 papUlse of the stigmata, thence passing down the ordinary stigmatic channels to arrive 

 at the ovules. As the stegium is contrived so that nothing can enter that hollow space, 

 it follows that the work of fertilization (as in the Apocynacece) is here effected without 

 the aid of insect agency. 



The apparent similarity in the raode of suspension of the pollen-masses in Orchidacece 

 has suggested an aifinity between that family and Asclepiadacece ; but such an approxi- 



» Linn. Trans, xvi. p. 734, tab. 34. figs. .4-6, tab. 35. figs. 7-11. 



b2 



