530 ON THE ORGANS AND MODE OF FECUNDATION
730] place through tlie whole length of the inner edge of the mass, which, as in all the genuine species of Hoya, is truncated and pellucid/ But I have not yet been able so to place the mass as to produce a cord of tubes communi- cating with the stigma, nor can I at present conjecture how this is to be effected.
I shall conclude with some observations equally relating to both the families that have been treated of.
It is in the first place deserving of remark, that while Asclepiadese and Orchidese so widely differ in almost every other respect, there should yet be an obvious analogy between them in those points in which they are distin- guished from all other Phaenogamous plants.
It is unnecessary here to state the numerous and impor- tant differences existing between these two famihes : but it may be of some interest to make a few remarks on their points of agreement or analogy.
These are chiefly two : The first being the presence of 731] an apparently additional part, not met with in other fami- hes ; the second, the cohesion of the grains of pollen, and their apphcation in masses to the female organ.
With regard to the first peculiarity it may be observed, that there is no real addition made to the number of organs in either family, and that in both famihes the apparent
^ In the tubes of Hoya carnosa I have been able to confirm Professor Amici's observation with respect to circulation taking place in the bcyaux of the grains of pollen. In this case the membrane being very transparent, and the granules, before the tube has acquired any considerable length, not being so numerous as to obscure the view of the opposite currents, they were very distinctly seen.
I have also observed circulation in the pollen tubes in a few other cases ; especially in Tradescantia virginica, in which, while the tube was still very short, the circle partly existing in the tube was completed in the body of the grain. The circular current in grains of pollen before the production of the tube may likewise, in some cases, but not very readily, be distinguished, as in Lolium peremie.
It might perhaps be supposed that the molecular motion, which in a former essay I stated I had seen within the body of the grain of pollen, might have been merely an imperfect view of the circulation of granules, and such I am inclined to think it really was in Lolium perenne.
I have, however, also'very distinctly seen within the membrane of the grain of pollen in some species of Asclepias, vivid oscillatory motion of granules without any appearance of circulation.
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