532 ON THE OEGANS AND MODE OF FECUNDATION 



In Asclepiadese 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 tubes through the cavity of the stigma in Orchideje, 

 the evidence appears to me unquestionable. With respect, 

 hovfever, to the origin of the cords formed of similar tubes, 

 so numerous and so regularly arranged in the cavity of the 

 ovarium, and vi^hich 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 1 have observed in Didymocarpus, Ipomopsis, and in 

 AUamanda, 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 Orchidese 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- 

 rinae, 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 arguments which might be adduced in favour of 

 this, the generally received opinion, would 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 what 

 manner the tubes themselves can operate. On the other 

 hand, their great diminution in number, or even total dis- 

 appearance, in Asclepiadese and Orchidese, long before the 

 ' See Additional Observations. 



