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 Asclepiadee 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 Asclepiadez I shall only observe, that I consider the evi- 
dence complete; but in Orchidez it may be admitted that it is 
not altogether so satisfactory." Of the descent of pollen tubes 
through the cavity of the stigma in Orchidez, 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 Orchidez 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 Scrophularine, Cistinez, 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. 
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