of the Structure of the Pollen Granule. 101 



of the Onagrarice and Proteacece,t2ike& part in this formation, 

 it is only in a much less degree than the rest of the surface. 

 I cannot therefore adopt the opinion of Mr. Robert Brown, 

 who especially attributes to these papillae the secretion of the 

 viscous matter." I have shown that the external membrane 

 does not enter into the structure of the papillje, and there- 

 fore in no instance can they have any participation in the for- 

 mation of the fluid referred to. 



Mohl considers the internal membrane of the pollen gra- 

 nule to be a simple homogeneous structure, and assigns to it 

 the office of secreting the fo villa, a function which it appears to 

 me to be very doubtful that it really performs ; for in watch- 

 ing the progress of the formation of the pollen, the existence 

 of the fovilla will be detected before any trace of the presence 

 of the enveloping membrane can be discovered. This mem- 

 brane, unlike perhaps all others which enter in the construc- 

 tion of the pollen granule, forms a closed cell, and is analo- 

 gous to a primitive vegetable cell. 



The interesting and remarkable facility, first discovered by 

 M. Dutrochet, possessed by all organic membranes, whether 

 vegetable or animal, of absorbing water or any other fluid of a 

 less dense nature than that contained within their cavities, is 

 thought, by M. Dutrochet, to pertain in a very high degree to 

 the internal membrane of the pollen granule. For my own 

 part, I do not see how a similar facility can be denied the ex- 

 ternal and other membranes of the pollen granule, since the 

 water absorbed by the internal must necessarily, in many cases, 

 pass through the outer before arriving at the inner membrane, 

 and since it is the presence of the fluid which gets between the 

 membranes which assists powerfully in causing the expulsion 

 of the pollen tubes, from the pressure which it produces on the 

 internal membrane. 



This absorption of fluid by the investing membranes of the 

 pollen granule sometimes takes place with such force as to oc- 

 casion the rupture of the internal membrane, and according 

 to Mohl, of that also of the thin part of the external ; for it is 

 only in this way, if Mohl's views are correct, that the pollen 

 tubes can emerge. In the dilute mineral acids this effect is 

 very frequently produced, but not always ; and when it does 

 occur, the effusion and coagulation of a portion of the fo- 

 villa, which often assumes a cylindrical and tortuous form, is 

 the result, and with respect to which Mohl gives the follow- 

 ing caution : — " We ought not to confound this mass, as has 

 been done by M. Meyen, with the internal membrane issuing 

 in the form of a tube ; the flrst is distinguished easily from 

 the second by its irregular form. M. Fritzsche, falling into 



