Fecundation in Orchidece and Asclepiadea. 721 



1829* has given a very interesting account of the structure of 

 the pollen masses in Asclepiadeae, from observations commenced 

 in 1825, and others made in 1828. 



In this account he describes the pollen mass as consisting of 

 a proper membrane bursting in a regular manner, the cavity- 

 being not cellular but undivided and filled with grains of pollen, 

 each grain having a cauda or cylindrical tube often of great 

 length, and all these tubes being directed towards the point or 

 line of dehiscence. This appendage or cauda he considers 

 analogous to the boi/au of Amici and Brongniart, differing 

 however in its forming an essential part of the grain in Ascle- 

 piadeae ; whereas in other families the application of an external 

 stimulus is necessary for its production. 



He is entirely silent as to the manner in which these caudate 

 grains communicate with or act upon the stigma ; and does not 

 in any case remark, — what must, I think, have been the fact, at 

 least in several of the plants in which this structure was observed, 

 and especially in those with pendulous pollen, — that the mass 

 examined was no lono-er in the cell of the anthera, but had been 

 removed and probably applied to some part of the stigma. 



In the month of July last I examined several species of As- 

 clepias, with reference to Mr. Bauer's drawings and Dr. Ehren- 

 berg's account of the pollen ; — the first object, therefore, was to 

 ascertain the structure of the pollen mass. 



Although on this subject my earliest observations essentially 

 agreed with Mr. Bauer's figures of the mass, which represent it 

 as having a subdivided cavity with a grain of pollen in each 

 cell ; yet a further examination had led me to adopt the opinion 

 of Treviranus and Ehrenberg, who describe its cavity as being 

 undivided and filled with distinct grains. 



* Linnaa iv. p. 94. 



4 z 2 I was 



