532 ON THE ORGANS AND MODE OF FECUNDATION
111 Asclepiacleae I shall only observe, that I consider the evidence complete ; but in Orchidese it may be admitted that it is not altogether so satisfactory. Of the descent of pollen tabes through the cavity of the stigma in Orchidese, the evidence appears to me unquestionable. With respect, however, to the origin of the cords formed of similar tubes, so numerous and so regularly arranged in the cavity of the ovarium, and which are in contact with surfaces not alto- gether incapable of secretion, it might perhaps be alleged, either that they wholly originate from the supposed con- ducting surfaces, or that they consist of a mixture derived from both sources.
That mucous threads, or capillary tubes, in most respects similar to pollen tubes, and certainly altogether belonging to the style, exist in some plants, there is no doubt ; and such I have observed in Didymocarpus, Ipomopsis, and in Allamanda, before the application of the pollen to the stigma. I am still, however, of opinion, that those found in the cavity of the ovarium in Orchideas are really derived from the pollen -^ an opinion which receives some con- firmation from the manifest descent of the pollen tubes in the style in many other families, as in several Scrophula- rinse, Cistinese, Viola, and Tradescantia.
The second question is. Whether the granules originally filling the grain of pollen, and which may often be found in the tubes, especially in their nascent state, both in these and in many other families, are the essential agents in the 733] process of fecundation ; the tubes being merely the channels conveying them to the organ or surface on which they are destined to act.
The argunients Avhich might be adduced in favour of this, the generally received opinion, w^ould probably be the variety in the form and size of the granules in different plants, with their great uniformity in these respects in the same species, added to the difficulty of conceiving in w hat manner the tubes themselves can operate. On the other hand, their great diminution in number, or even total dis- appearance, in Asclepia(leae and Orchidea?, long before the ' See Additional ObservatiQiis.
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