IN ORCHIDEiE AND ASCLEPIADEiE. 533 



margins of the areolae of the inner surface ofthe mass, added 

 to the facts which had originally led me to adopt Mr. Bauer's 

 view, determined me to re-examine the subject. 



The result of this examination, made on specimens of 

 Asclepias pliytolaccoides and purpurascens, but especially the 

 former, proved that the mass in these species is really cellu- 

 lar in all stages, as Mr. Bauer has represented it in A. 

 ctirassavica, and that in the advanced flower-bud, as in the 

 expanded flower, the cells may be seen, though not without 

 difficulty, after their grains are removed. 



The pollen mass in several species of Asclepias, particu- 

 larly ill Asclepias ^^^ii/tolaccoides^ (and in A. curassavica, 

 as figured by Mr. Bauer), consists of cells disposed in three 

 series parallel to its sides, the middle series being often 

 more or less interrupted. 



The cells of the outer layer of each side have their oppo- 

 site walls very unequal both in colour and thickness. The 

 outer wall of each of these cells, which is formed by one of 

 the areolae of the surface, is of a deep yellow colour, nearly 

 opaque, and of such thickness as to prevent external burst- 

 ing ; the inner is of a paler yellow, semi-transparent, and so 

 much thinner as to determine internal rupture, which in 

 these cells, after the production of the tubes, seems to take 

 place without regularity, and to such an extent, that after 

 the removal of the grain the remains of the inner wall are ws 

 not very readily distinguishable. 



Sections of the mass, indeed, both transverse and longi- 

 tudinal, exhibit an appearance of cellularity ; but there is 

 here a source of fallacy, unless the contained grains are also 

 visible in the section ; and the best proof of its being cel- 

 lular 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 

 throughout, and on the production of the tubes burst in a 

 definite manner towards the convex Tsdge 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 



I Tab. 35, fig. 8. 



