R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 131 



impregnated germ cell, and tlie loose cells of the fleshy fruit of 

 cherries or strawberries supply material for its observation. 



1. The Cell Membrane. — Primary cell membrane — Primor- 

 dial utricle of Mohl — is recognised by its single strong contour, 

 the outline being smooth, or granular from pressure of contents on 

 the delicate membrane ; in the germ cell it is extremely thin, so 

 that water dissolves it, but in endosperm cells it is stronger, and 

 by bursting the utricle in water, the contents escape, and a clear, 

 slightly folded bladder is left. By chemical reagents we obtain 

 distinctive coloration of the primordial utricle, watery solution of 

 Iodine' tinges it deep yellow, indicating its albuminous nature ; dilute 

 nitric acid and liq. ammonia a similar colour ; sugar and sulphu- 

 ric acid rose red ; sulphate of cojDper and liq. potassa violet. The 

 action of these reagents, as well as of syrup, alcohol, &c., which 

 remove water, cause the cell membrane and its contents to shrivel. 

 In cells where the cellulose case is also formed, the two membranes 

 are so blended as to be optically indistinguishable ; they may, how- 

 ever be isolated by endosmotic media, as sol. of sugar, salt, or 

 iodine, and we see the primordial cell, like a little sac, closed on all 

 sides, lying sharply defined within the cellulose case. The cell 

 membrane alone takes an active part in the life phenomena of the 

 cell, the cellulose case does not do so in any way. 



2. The Cellulose Case. — Cell case, cell wall, or cell mem- 

 brane of botanists — is, next to the enlargement of the primordial 

 cell, the earliest product of the organic vegetable fomiative prin- 

 ciple, and adapted to serve as a protective covering to the cell. It 

 is also the most enduring part of it, for after all active life has 

 ceased in the nucleus and cell membrane, the cellulose case may 

 remain entire for an indefinite period. 



For the study of the cellulose case the best adapted is the free 

 cell, as when combined into tissues their super-position and con- 

 tents greatly interfere with distinct observation ; the first envelope, 

 however, which separates after the cell-membranes does not become 

 the cellulose case, but undergoes further transformation; for certain 

 purposes, however, as the determination of structure, surface- 

 markings, &c., the separated tissue elements supply the best 

 material. As soon as the cellulose case has separated from the 

 primordial membrane and become firm, it becomes evident under a 

 ])ower of 350 diameters, by its double bounding line, and appears 

 quite hymogeneous. Chemically, it agrees in constitution with the 



K 2 



