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 IN ORCHIDEÆ AND ASCLEPIADEÆ. 527

opportunities of examining during the summer. For in those species in which the pollen mass was not found transferred from its original position to the fissure, and in contact with the base of the style, no doubt by means of insects, it was not difficult to place it there ; and in doing so I never failed to obtain the same results.

I now turned my attention to the base of the stigma, expecting to find there such a modification of surface as might serve to account for the rupture and production of the tubes in the mass brought in contact with it. I have, however, in no case been able to observe the slightest :727 appearance of secretion, or any difference 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 1 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 prominent point of the convex edge, and to this part it is generally confined : it is sometimes, however, found ex- tending through the greater part of its length.

On carefully examining the convex edge, and more particularly its most prominent portion, I have not been able to observe 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 prominent 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.