Fecundation in Orchidee and Asclepiadea. 1723 
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 Asclepiadez, 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. i 
Cells. 
