ROBERT BROWN 93 



mis of a great part of this family (Orchidaceae), especially 

 of those with membranaceous leaves, a single circular 

 areola, generally somewhat more opaque than the mem- 

 brane of the cell, is observable. This areola, which is 

 more or less distinctly granular, is slightly convex, 

 and although it seems to be on the surface is in reality 

 covered by the outer lamina of the cell. There is no 

 regularity as to its place in the cell ; it is not infrequently, 

 however, central or nearly so. ... This areola, or 

 nucleus of the cell, as perhaps it might be termed, is not 

 confined to the epidermis, being also found in the paren- 

 chyma or internal cells of the tissue. In the compressed 

 cells of the epidermis the nucleus is in a corresponding 

 degree flattened, but in the internal tissue it is often 

 nearly spherical, more or less firmly adhering to one of 

 the walls and projecting into the cavity of the cell. . . 

 This nucleus of the cell is not confined to Orchideae, but 

 is equally manifest in many other Monocotyledonous 

 families ; and I have even found it, hitherto however in 

 very few cases, in the epidermis of Dicotyledonous 

 plants. 1 ' In these simple words Brown announced the 

 discovery of the cell nucleus on which a whole library of 

 memoirs was to be written in the years to follow, both on 

 its minute structure and on the complex changes it 

 undergoes during the process of cell division. 



There is another but in itself relatively unimportant 

 observation made by Brown and now always known by 

 his name, viz. " Brownian movement." This term, as 

 you may know, is applied to the oscillation of minute 

 particles in a fluid as seen under the microscope. These 

 movements are purely physical, as indeed Brown himself 

 ultimately concluded, seeing that they take place among 

 inorganic particles of sufficiently small size when suspended 

 in water. Various explanations have been offered to 

 account for this vibration, the one commonly accepted 

 being that it is due to the perpetual bombardment of 

 these particles by the molecules of the medium, but for 



