IN ORCHTDEiE AND ASCLEPIADE/E. 531
addition consists in a modification or prodnction of the stigma ; the modified part of which loses the proper func- tion of that organ.
This production of the stigma, — which is generally present, and wanting only in certain Orchidcae, where its place is sometimes snpplied by an analogous modification of the male organ, — though differing very remarkably in appearance in the two families, agrees in being originally distinct from the pollen masses, and in the advanced stage becoming firndy attached to them ; in adhering but slightly to the point of its formation after the attachment to the pollen takes place ; and in being so constructed as to be readily removed by insects from its original position along with the pollen masses.
As to the second point of agreement ; namely, the co- hesion of the grains of pollen into masses of considerable size, and the application of these masses to the stigma, — it is obviously connected with that which might perhaps be termed a third pecuharity ; the apparent necessity for an unusual number of pollen tubes which are to act in concert ; in the one family to penetrate to and regularly arrange themselves in the cavity of the ovarium -^ in the other to open a communication with the stigma, and then to pass along a non-secreting surface, until they arrive at a distant point, where they are to be introduced into the cavity or body of the style.
With respect to the agency of Insects in fecundation in those two orders, there can be no doubt that it is very fre- quently employed in Orchideae ; at the same time there are evidently cases in that family in which, from the relative [732 position of the organs, the interposition of these agents is not always required, l^ut in those Asclepiadeae at least that have been fidly examined, the absolute necessity for their assistance is manifest.
Two questions still remain.
The first regards the ])roof of the actual penetration of the pollen tubes into the cavity of the ovarium in both families.
' See Additional Observations.
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