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 524 ON THE ORGANS AND MODE OF FECUNDATION

considered, the inner membrane of the grain of pollen, whose onter membrane is formed by the cell itself; and the tenacity of this onter membrane is such that it may easily be removed from the inner without further apparent rupture.

These central grains, thus covered by their respective cells, may readily be distinguished, by their pale yellow colour and a certain degree of opacity, from the naked grains or inner membranes, which, like their tubes, are entirely colourless, and transparent/

In Asclepiadeae, therefore, it may be said that the greatest development of the pollen grain exists; namely a grain having an undivided cavity, whose membranes are entirely distinct, and the pollen tubes of which seem to possess the highest degree of vitality yet met with.

In the perfectly developed state of the pollen mass, the grain, considered as distinct from its outer membrane or containing cell, is nearly round, but slightly and obtusely angular, much compressed, with an undivided cavity, and exhibiting no indication of its being composed of four or 724] any other number of united cells. Its membrane is trans- parent and colourless, made up of two united coats, and the cavity is filled with spherical granules of nearly uniform size, among which a few oily particles are occasionally ob- servable.' In this state no appearance or indication of the tubes or appendages described by Dr. Ehrenberg is found.

On the 16th of July, in repeating my examination of Asclejnas j^urpiirascens,^ I observed in several flowers one or more pollen masses removed from their usual place, namely the cell of the anthera, and no longer fixed by the descending arm to the gland of the stigma, but immersed in one of the fissures formed by the projecting alae of the antherae, and in most cases separated from the gland, a small portion of the arm or process, generally that only below its flexure, remaining attached to the mass.^

1 Tab. 35, fig. 9. = Tab. U, fig. 6; and tab. 56, figs. 3 and 13.

3 Tab. U. ' Tab. 35, figs. 2, 3, 4, and 7.

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