518 ON THE ORGANS AND MODE OF FECUNDATION
In 1793, Christian Konrad Sprengel, ^vho adopts the opinion of Jacquin Loth ^vith respect to the pollen masses and pentagonal stigma, further states, that this stigma has a secreting npper surface or apex, and is formed of two united bodies, each of which conveys to its corresponding ovarium the fecundating matter, consisting of the oily fluid which exudes from the surface of the pollen mass. He also considers insects as here essentially necessary in im- pregnation, which they eff'ect by extracting, in a manner particularly described, the pollen masses from the cells, and applying them to the apex of the stigma. And lastly, as extraordinary activity of the insect is necessary, or at least advantageous in the performance of this operation, that activity is, according to him, produced by the intoxi- cating secretion of the nectaria.-^
In 1809, an essay on Asclepiadese was published in the first volume of the Memoirs of the Wernerian Natural History Society, in which one of my principal objects was to establish the opinion, more or less conjectural, of Adanson, 718] Uichard, Jussieu, and Schreber, respecting the struc- ture of the stamina and stigma. With this view I appealed to the remarkable fact, that in the early state of the flower- bud the pollen masses are absolutely distinct from the glands and processes of the stigma, to which they in a more advanced stage become attached. This proof of the real origin of parts I then believed to be entirely new. It has, however, been already seen that the fact was noticed by Gleichen, and it will presently appear that it w^as also well kno^vn to another original observer.
In the essay referred to, I had not very minutely ex- amined the texture of the pollen mass, and in true Ascle- piadeae I had failed in ascertaining its real internal struc- ture ; not having been then aware of the existence of the included grains of pollen, but believing, until very lately, that the mass in its most advanced state consisted of one
! It may here be remarked, that the prevailing form of inflorescence in Asclepiadese is well adapted to this economy ; for the insect so readily passes from one corolla to another, that it not unfrequently visits every flower of the umbel.
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