130 R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



limit of a single cell, is comprised tlie whole life history of the 

 individual — birth, nutrition, growth, reproduction, death. 



Neither can any distinction be found between the primitive animal 

 and vegetable cell, and since all organized creation thus originates, 

 the study of a cell becomes invested with an importance that can 

 scarcely be over estimated, and with that study we must commence 

 our investigations, if wo would rightly understand the wonderful 

 changes brought about by the vital processes in the multiplication 

 and metamorphosis of colls consequent on growth. 



Structure of the Plant Cell. 



Each individual cell is an independent microscopic organism, 

 which, according to the latest theory, is in its primitive state, like 

 the animal cell, deficient of any enclosing membrane, and consists 

 essentially of a little lump of protoplasm, enclosing the cell 

 nucleus. 



The constituents, however, of the primordial cell are usually 

 regarded as being — 



1. The cell membrane, composed of an albumenoid substance. 



2. The cell contents, separable into protoplasm and cell sap. 



3. The cell nucleus, a small body suspended in the protoplasm, 

 and composed of smaller nucleoli. 



At a later stage of its life, the primordial cell becomes sur- 

 rounded by a second membrane, a case or capsule, the cellulose case, 

 composed of carbon, hydrogen, and oxygen, the presence of which 

 distinguishes histologically the plant cell from the higher animal 

 cell. 



We meet with the vegetable cell in its first stage of develop- 

 ment, in the zoospores of algse, the spermatozoids of mosses and 

 ferns, as the germ cell of all plants, as the first endosperm cells in 

 the unimpregnated embryo sac of many families, as Liliacese, 

 Umbelliferse, Papilionacese, &c., and as young pollen cells in the 

 cells beginning to form pollen by cross-division, which are best 

 observed by section of young buds. 



The next step in the investigation of the free cell, is the obser- 

 vation of the stage when it has formed the first envelope of cellu- 

 lose, the primary cellulose case. This condition, hitherto the basis 

 of definition of the living vegetable cell, is truly only a certain 

 stage of perfection of the same, and is at once evident by the double 

 contour of the circumference, and the firmness of the whole ; the 



