730 Mr. Brown on the Organs and Mode of 



through the 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 communicating 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 Ascle- 

 piadeae and Orchideee so widely differ in almost every other 

 respect, there should yet be an obvious analogy between them 

 in those points in which they are distinguished from all other 

 Phsenogamous plants. 



It is unnecessary here to state the numerous and important 

 differences existing between these two families : but it may be 

 of some interest to make a few remarks on their points of agree- 

 ment or analogy. 



These are chiefly two : The first being the presence of an 



* In the tubes of Hoya carnosa I have been able to confirm Professor Amici's ob- 

 servation witli respect to circulation taking place in the boyaux 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 cur- 

 rent in grains of pollen before the production of the tube may likewisCj in some cases, 

 but not very readily, be distinguished, as in Lolium perenne. 



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 appear- 

 ance of circulation. 



apparently 



