Fecundation in Orchidea and Asclepiadea. 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 



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