528 OM THE ORGANS AND MODE OE EECUNDATION 



and applied to tbe 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 generally in every grain in the mass. But 

 as there are no visible conductors of this stimulus within 

 the mass, it must either be supposed to be propagated 

 from one cell to another, or conveyed from the prominent pas 

 point of the edge to every other part of the surface of the 

 covering itself. 



To ascertain whether contact of the convex edge of the 

 pollen mass with this point of the stigma Avas absolutely 

 necessary for the rupture of the mass and the production 

 of tubes, I in the first place introduced a mass into the 

 fissure, but with its convex edge outwards. In this posi- 

 tion no change whatever took place. 



I next removed one of the glands of the angles of the 

 stigma, and applied the convex edge of a mass to the 

 surface thus exposed, which even in this stage — to facilitate 

 the removal of the gland by insects — continues to secrete. 

 In this case, dehiscence and protrusion of pollen tubes did 

 follow, more slowly, however, and less completely, than 

 when brought in contact with the non-secreting base. 



On applying the pollen mass of one species of Asclepias 

 to the base of the stigma of another, the usual changes 

 generally took place ; but still, as it seemed, less perfectly, 

 and only after a longer interval. 



Pollen masses oi Asclepias purpurascens being applied to 

 the stigma of Epipacfis pdiistris, and immersed in its 

 viscid secretion, the dehiscence, contrary to expectation, 

 not only took place, but even more speedily than usual, 

 that is within twenty-four hours. Some of the grains were 

 also found discharged from the mass unchanged, while 

 others, both discharged and still inclosed, had begun to 

 produce tubes. 



The greater number of these observations were also 

 made with A. pJiytoIaccoides, which, on account of the 

 greater size of its flower, I at first preferred. I found, 

 however, with reference to such experiments, an objection 

 to employing this species, arising from the great excitability, 



