Fecundation in Orchidece and Asclepiadea . 729 



gradual decay of the flower, where the parts remain soft, the 

 rupture and protrusion of tubes took place while the mass was 

 still in its original position, immersed in the cell of its an- 

 thera*. The tubes produced in this situation often acquire a 

 great length, but coming, immediately on their protrusion from 

 the mass, in contact with the membrane of the anthera, their 

 course is necessarily altered ; and in their new direction, which 

 is generally upwards, they not unfrequently arrive at the top of 

 the cell, or even extend beyond it. 



In addition to the several species of Asclepias already referred 

 to, Cynanchum (Vincetoxicum) nigrum is the only plant of this 

 family in which I have observed the whole of the appearances ; 

 namely, the rupture of the mass, the production and protrusion 

 of the pollen tubes, their union into a cord, with the course and 

 entrance of this cord into the cavity of the style. 



The present essay therefore, as far as regards this family, 

 might with greater propriety have been entitled, " On the mode 

 of impregnation in the genus Asclepias." It seems, however, 

 allowable to conclude, that in all the genera having pendulous 

 pollen masses, the same economy, slightly modified perhaps in 

 some cases, is likely to be found. But among those with erect 

 pollen masses, there are several in which more considerable dif- 

 ferences may be expected. Of this section of the family I have 

 hitherto liad the opportunity of submitting only one plant to 

 careful examination, namely, Hoya carnosa ; and even here my 

 observations are incomplete. 



In Hoya carnosa I have never found the pollen tubes pro- 

 duced, or masses ruptured, while remaining in their original 

 position ; but I have succeeded in producing these effects by 

 bringing them in contact with certain parts of the corona. 



The rupture and protrusion of pollen tubes, then, take place 



* Tab. 35. fig. 11. 



5 A 2 through 



