MR. T. Н. CORRY ON ASCLEPIAS CORNUTI. 201 
the free portions of the two styles not being even excepted. This tissue covering the 
inferior surface is directly continuous, on the one hand, with the mass of tissue lining 
the cavities of the two styles in the region of the sutures, and on the other with tissue 
forming for some distance upward the central portion of the style-table between the two 
closely approximated fibro-vascular bundles. The papillar tissue of the stigma just 
described evidently escaped Brown's observation, since he expressly states that when he 
examined the base of the stigma he was “ in no case able to observe any difference whatever 
in texture between that part and the general surface of the stigma” *. Nor could he 
discover the slightest appearance of secretion, though he expected to find there such a 
modification of surface as might serve to account for the rupture of the pollinium and 
protrusion of the pollen-tubes. Its discovery was due to Adolphe Brongniart, and was 
afterwards independently confirmed by Hermann Schacht, though the observations of both 
appear to have been overlooked. But though five shallow radial furrows exist on the upper 
surface of the staminal column and papillar tissue on the inferior surface of the stigma 
above them, the tissue of the two parts at these points is never continuous, though 
naturally the papille project obliquely downwards into the furrow, and so, apparently, 
the two parts are strongly applied together and adherent (fig.13). Robert Brown, Brong- 
niart, and others regarded this adherence and application as an exceedingly close and inti- 
mate one. The latter writer tells that it takes place in such a way and so strongly that 
no means of communication is left between the pollinium and the base of the style-table, 
but these parts are completely isolated, while Brown makes use T of the word “ union" to 
imply the extent to which he believed it had taken place. Both authors speak of the 
skein of pollen-tubes proceeding from the ruptured pollinium as insinuating itself en masse 
between the base of the style-table and the upper edge of the staminal column by sepa- 
raling the one from the other at this point, and so opening a channel. This, it is true, 
the skein of pollen-tubes does to а certain extent; but I have never had the least difficulty 
in finding, when I looked carefully for it, the partially unclosed entrance to the radial 
groove, even in stages long before the production of the skein of tubes. Consequently the 
skein of tubes does not, I think, require forcibly to separate the closely adherent parts 
and so effect an entrance for itself, as these writers believed. Brown was unable to find 
that the cells bordering the course of the skein of tubes along the inferior angle, or at the 
point to which the pollinium was attached, were secretory; and I have also repeatedly failed 
to find any evidence that they produce an external secretion. Notwithstanding this, I think 
that the cellulose walls of the papilliform cells are so permeated with liquid in every part of 
their substance that the immediate proximity to, or actual contact with, these at the par- 
tially open end of the radial furrow aids and, in part, determines the rupture of the 
pollinium at the most prominent point of its convex edge and the production of the skein 
of pollen-tubes. This opinion is based partially on experiments which I have made on 
mature pollinia immediately after their extraction by immersing them in water, dilute 
glycerine, strong glycerine, and solutions of sugar of different strengths, and partially 
also on a few experiments made by Robert Brown. 1 have invariably found that rupture 
of the pollinium occurred at its convex edge when it was immersed for some time in dilute 
з Misc. Bot. Works, vol. 1. р. 527; Linn. Trans. vol. xvi. р. 727. + Misc. Bot. Works, vol. i. р. 526. 
