Fecundation in Orchidee and Asclepiadea. 129 
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 had the opportunity of submitting only one plant to 
careful examination, namely, Hoya carnosa ; and even here my 
observations are incomplete. 
In Hoya carnosa lhave 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. 
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