Fecundation in Orchidee and Asclepiadee. 727 
able to observe the slightest appearance of secretion, or any dif- 
ference whatever in texture, between that part and the general 
surface of the stigma. 
The bursting of the mass in Asclepias is uniformly on the 
more rounded edge; and this, it may be observed, is the inner 
edge or margin of the mass, with reference to the cell of the 
anthera in which it is formed; and I may further remark, that 
in the only case in which I have hitherto observed dehiscence 
in an erect pollen mass, namely, in Hoya carnosa, it also takes 
place along the inner margin. 
In Asclepias the bursting always commences at the most pro- 
minent point of the convex edge, and to this part it is generally 
confined : it is sometimes however found extending through the 
greater part of its length. | 
On carefully examining the convex edge, and more particu- 
larly its most prominent portion, I have not been able to ob- 
serve in it any change or peculiarity of texture, or even any 
obvious difference in the form of the meshes of the reticulated 
surface. Notwithstanding this apparent want of secretion in the 
base of the stigma, and of difference of texture in the covering 
of the mass of pollen at the point where it comes in contact 
with that organ, it must still be supposed that there is some 
peculiarity both in the surface of the stigma and in the promi- 
nent edge of the mass, on which the effects in question depend. 
These effects are indeed very remarkable; the stimulus here 
supposed to be derived from the surface of the stigma, and ap- 
plied to the prominent point of the convex edge of the pollen 
mass, producing its appropriate action not only in those cells or 
grains of pollen in immediate contact with that point, but gene- 
rally in every grain in the mass. But as there are no visible con- 
ductors of this stimulus within the mass, it must either be sup- 
posed to be propagated from one cell to another, or conveyed 
SNOL? XVI: 5A from 
