494 PHYSIOLOGY. 



and treated with iodine and sulphuric acid, the cellulose becomes 

 stained blue, the protoplasm yellow, and threads of the latter may 

 be seen passing from one cell to the other. 



In the formation of the ultimately free cells composing; pollen-grains 

 and of the spores of the higher Cryptogamia, the cells are liberated from 

 the parent cells by solution of the* wall of the latter. The outer layers 

 of the cell-wall indeed often assume a different appearance from the inner 

 ones ; and, as in the case of the pollen-tube, the outer coat dies and splits 

 as the inner coat grows into a tube. A still more curious phenomenon 

 occurs in the process of conjugation, where two cells coalesce by complete 

 union of their walls. The last cases appear related in some degree to the 

 origin of the gelatinous coats of the Palmellece and other Confervoids, 

 which are probably produced by the disintegration of the walls of parent 

 cells, which become softened and swell up as the new generations of cells 

 are formed in their interior. 



Contents of the Cell. The solid cellulose structures forming 

 the persistent mass of vegetable tissues may be regarded as a 

 skeleton or framework; for the vital and chemical phenomena 

 exhibited by plants all depend, in the first instance, upon operations 

 which have their seat in the interior of the cells. The careful 

 investigation of the cell-contents is consequently of primary im- 

 portance in the study of Vegetable Physiology. 



The fundamental importance of the matters within the cell is not only 

 demonstrated by what we are enabled to observe taking place in the in- 

 terior of living cells, but, in certain of the lower plants, the vitalized con- 

 tents actually emerge from their confinement in the shell of cellulose (as in 

 the case of the so-called zoospores], move, perform all the functions of plant- 

 life, and exhibit in the course of their subsequent conversion into closed 

 motionless cells exactly the same power to form new cell-membrane as 

 takes place in ordinary cell-formation. 



Cell-sap, &c. The contents of the cell are partly more or less solid, 

 partly fluid. When substances exist dissolved in the cell-sap, they 

 are frequently out of the reach of microscopic observation, on account of 

 the minute quantities in which they exist, or from the want of suitable 

 reagents to ascertain their presence ; among these are the vegetable alka- 

 loids and similar products. The sugar, dextrine, mineral salts, &c. dis- 

 solved in the watery cell-sap do not readily admit of examination in this 

 way. The fluid colouring-matters, essential or fixed oils, resins, &c., on 

 the contrary, are readily observed, on account of their distinct physical 

 and chemical characters. This is still more the case with mineral or 

 organic salts which are sufficiently abundant to crystallize in the cell. 



But by far the most important of the contents of cells are certain 

 organized structures which are regularly met with in the cell-contents, 

 either universally or, with certain definite exceptions, at particular epochs 

 of the life of cells. These are the protoplasm, the nucleus, and chlorophyll* 

 corpuscles, which are albuminous or proteinaceousin their character, while 

 others are destitute of nitrogen, such as the starch-granules. 



