732 Mr. Brown on the Organs and Mode of 



in that family in which, from the relative position of the organs, 

 the interposition of these agents is not always required. But in 

 those Asclepiadese at least that have been fully examined, the 

 absolute necessity for their assistance is manifest. 



Two questions still remain. 



The first regards the proof of the actual penetration of the 

 pollen tubes into the cavity of the ovarium in both families. 



In Asclepiadeae 1 shall only observe, that I consider the evi- 

 dence complete ; but in Orchideae it may be admitted that it is 

 not altogether so satisfactory. Of the descent of pollen tubes 

 through the cavity of the stigma in Orchideae, the evidence ap- 

 pears 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 altogether incapable of secre- 

 tion, it might perhaps be alleged, either that they wholly ori- 

 ginate from the supposed conducting surfaces, or that they con- 

 sist 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 ova- 

 rium in Orchideae are really derived from the pollen*; an opinion 

 which receives some confirmation from the manifest descent of 

 the pollen tubes in the style in many other families, as in seve- 

 ral Scrophularinae, Cistineae, Viola, and Tradescantia. 



The second question is. Whether the granules originally fill- 

 ing 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 process 



* See Additional Observations. 



of 



