of the Structure of the Pollen Granule. 



97 



" In these plants the granular formation tends so to disappear, 

 that the external membrane presents only obscure little dots, 

 which have scarcely any resemblance with cells. Lastly, in 

 other kinds of pollen the grains disappear entirely, and the 

 membrane is almost completely smooth and uniform. Never- 

 theless, in most cases, when the pollen is observed in water, 

 we are able still to recognise, with the assistance of a good in- 

 strument, a very fine punctuation and a light yellow colour of 

 the membrane, from which circumstance it is very probable 

 that the external membrane is never perfectly homogeneous, 

 but contains always grains, however little distinct they may 

 be. All that has now been set forth appears to prove that the 

 comparison of the external membrane of the pollen granule with 

 a vegetable cell is altogether inexact, and that it ought to be re- 

 garded as an organ composed of cells, or of the rudiments of 

 cells, and of a homogeneous element which unites them ; and for 

 this reason also, to compare it, not with the simple membrane of 

 a vegetable cell, but with compound membranes, for example, 

 with the membranes of the ovule." 



I have stated elsewhere*, that the granular appearance some- 

 times presented by the external membrane of the pollen granule 

 frequently arises from the circumstance of the active 6C molecu- 

 lar bodies" being visible through the transparent coverings of 

 the granule, so conveying to the outer one a granular or dotted 

 aspect. That this appearance is often, if not always, decep- 

 tive, I am still of opinion, for additional reasons. Mohl states, 

 that upon the part which in the dry pollen is concealed by 

 the fold, the grains sometimes form groups scattered at great 

 intervals. Now the radiating bands, of generally a lighter colour 

 than the proper external envelope of the pollen granule, which 

 supply the place of the furrows on the immersion of the pol- 

 len in water, are produced by the protrusion of the internal 

 membrane through the fissures in the extine placed at the bot- 

 tom of the furrows, and provided for the free egress of the 

 pollen tubes ; and this membrane is admitted by Mohl, and 

 all observers, to be a simple structure. This, although a ne- 

 gative objection, is still a strong one. Again, in another place 

 Mohl observes, that in some cases it happens that a portion 

 of the extine is detached from the internal membrane, and 

 that this part presents the appearance of a uniform and co- 

 lourless membrane : this also goes to prove that the external 

 membrane is, in some cases at least, simple. I am, however, 

 still willing to admit, that in some few instances the external 

 membrane may really present a granular texture, and then 

 * In loc. cit. 



Ann.fr Mag. N. Hist. Vol.ix. H 



