524 ON THE ORGANS AND MODE OF FECUNDATION 



considered, the inner membrane of the grain of pollen, 

 whose outer membrane is formed by the cell itself; and 

 the tenacity of this outer membrane is such that it may 

 easily be removed from the inner without further apparent 

 rupture. 



These central grains, thus covered by their respective 

 cells, may readily be distinguished, by their pale yellow 

 colour and a certain degree of opacity, from the naked grains 

 or inner membranes, which, like their tubes, are entirely 

 colourless, and transparent.^ 



In Asclepiadeae, therefore, it may be said that the greatest 

 development of the pollen grain exists ; namely a grain having 

 an undivided cavity, whose membranes are entirely distinct, 

 and the pollen tubes of which seem to possess the highest 

 degree of vitality yet met with. 



In the perfectly developed state of the pollen mass, the 

 grain, considered as distinct from its outer membrane or 

 containing eel!, is nearly round, but slightly and obtusely 

 angular, much compressed, with an undivided cavity, and 

 exhibiting no indication of its being composed of four or 

 724] any other number of united cells. Its membrane is trans- 

 parent and colourless, made up of two united coats, and 

 the cavity is filled with spherical granules of nearly uniform 

 size, among which a few oily particles are occasionally ob- 

 servable.^ In this state no appearance or indication of 

 the tubes or appendages described by Dr. Ehrenberg is 

 found. 



On the 16th of July, in repeating my examination of 

 Asdepias purpurascens,^ I observed in several flowers one 

 or more pollen masses removed from their usual place, 

 namely the cell of the anthera, and no longer fixed by the 

 descending arm to the gland of the stigma, but immersed 

 in one of the fissures formed by the projecting alse of the 

 antherse, and in most cases separated from the gland, a 

 small portion of the arm or process, generally that only 

 below its flexure, remaining attached to the mass.* 



' Tab. 35, fig. 9. "- Tab. 34, fig. 6 ; and tab. 56, figs. 3 and 13. 



^ Tab. 34. " Tab. 35, figs. 2, 3, 4, and 7. 



