IN ORCHIDE.E AND ASCLEPIADE^. 523
margins of the areolae of the inner surface of the mass, added to the facts which had originally led nie to adopt Mr. Bauer's' view, determined nie to re-examine the subject.
The result of this exauiination, made on specimens of Jsclej)ia.s p/i i/iohtccoides ^\\{\ /jurpfrascc/fs, 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 J. Cffras-savlca, and that in the advanced flower-bud, as in the expanded flower, the cells may be seen, though not without ditlicnlty, after their grains are removed.
The pollen mass in several species of Asclcpias, particu- larly ill Ai^dcpias plnjlolaccoitlcs^ (and in A. cifrassavica, 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, Avhicli in these cells, after the ])roduction of the tubes, seems to take place Avithout regularity, and to such an extent, that after the removal of the grain the remains of the inner wall are [7-^3 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 edi^e 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
' T:ib. 35, fi}?. 8.
�� �
