THE ANDRCECIUM. 147 



(Althea.) In most plants these grains are free amongst them- 

 selves ; but in the Fuchsia and ^Enothera biennis, or evening 

 primrose, they are held together by slender threads, and in 

 other genera they adhere together in masses called pollinia. 



Pulverulent pollen. This is its most general aspect and 

 disposition. Pollen cells are ordinarily composed of two 

 membranes, which are distinguished as external and internal. 

 The interior of the cells is filled with a mucilaginous fluid 

 matter, containing granules, named fovilla. The exterior 

 membrane of the pollen cell, denominated the extine, (cxto, 

 to stand out,) is thick, firm, and is readily ruptured by dis- 

 tension. It is this membrane which is covered with papillae 

 or granulations, the surface of the pollen being rarely smooth. 

 It is applied immediately on the internal membrane, or intine, 

 (intus, within.) This membrane is thin, transparent, very 

 extensible, and without any appreciable organization. 



The mucilaginous fluid and granular matter in the interior 

 of the pollen cells has been the object of a great deal of discus- 

 sion amongst physiologists. The fovilla exhibit very marked 

 movements in the fluid where they swim. These movements, 

 it was at first thought were spontaneous, and the pollenic 

 granules were supposed to be assimilated by them to the 

 zoosperms of animals. But the analogy has been completely 

 destroyed by an examination of the chemical nature of these 

 bodies, which are nothing but grains of starch, turning blue 

 with iodine, and showing all the characters of the fecula taken 

 from the other parts of the plant. This observation is due to 

 M. Fritsch of Berlin, who published in 1832 and 1833 two 

 interesting dissertations on pollen. 



Solid pollen is that in which the grains instead of being 

 distinct are united together in masses, which in general take 



