Fecundation in Orchidece and Asclepiadece. 723 



the grain the remains of the inner wall are not very readily 

 distinguishable. 



Sections of the mass indeed, both transverse and longitudinal, 

 exhibit an appearance of cellularity ; but there is here a source 

 of fallacy, unless the contained grains are also visible in the sec- 

 tion : and the best proof of its being cellular is derived from the 

 state of the central or middle series after the bursting of the mass. 



The cells of this central layer are of equal thickness through- 

 out, and on the production of the tubes burst in a definite man- 

 ner towards the convex edge of the mass, and at the same time 

 generally separate from each other. They continue however 

 to inclose the grain, or, as it may be considered, the inner 

 membrane of the grain of pollen, whose outer membrane is 

 formed by the cell itself ; and the tenacity of this outer mem- 

 brane 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 mem- 

 branes, 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 de- 

 gree 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 cell, 

 is nearly round, but slightly and obtusely angular, much com- 

 pressed, with an undivided cavity, and exhibiting no indication 

 of its being composed of four or any other number of united 



* Tab. 35. fig. 9. 



cells. 



