492 PHYSIOLOGY. 



them with water and applying iodine, a blue colour is produced like 

 that appearing in nascent cellulose or in tolerably new tissues under the 

 influence of sulphuric acid. 



It remains to be ascertained whether these processes alter the com- 

 position of the cell-membranes, or merely remove infiltrated matters of 

 nitrogenous composition. The latter view is supported by the fact that, 

 in imperfectly prepared objects, some of the more resisting layers appear 



Creen, which would seem to result from an optical combination of the 

 lue of the cellulose with the yellow of an infiltrated matter. At the 

 same time it must be noticed that the cellulose is brought into a condition 

 approaching that of starch, only normal in nascent membranes and in the 

 semisolid deposits of " amyloid " above mentioned. Fremy, as above 

 stated, considers that there are other substances besides cellulose entering 

 into the composition of vegetable cell-walls. Tt*ue cellulose forms the 

 cell-wall of the cellular tissue of bark, fruits, roots, &c., and is soluble in 

 ammcniacal copper oxide, made by dissolving copper filings in caustic 

 ammonia. Paracellulose is found in the cells of the pith, the epidermis, 

 the medullary rays, &c. ; it is soluble in the copper solution, but only 

 after special treatment. Fibrose is the constituent of the wood-cells, and 

 is insoluble in the copper solution, except after special treatment, but 

 soluble in strong sulphuric acid. Vasculose, the substance of which vessels 

 are formed, is insoluble in hydrochloric and sulphuric acids and in the 

 copper solutions, but soluble in boiling caustic potash. It is coloured by 

 anilin with a little sulphuric acid. 



Inorganic Constituents. Cell-membrane in most cases contains 

 a certain amount of inorganic matter ; but this is probably attribu- 

 table in general to its being saturated with the watery cell- sap, in 

 which various salts exist in solution. In particular cases, however, 

 there is a special deposition of inorganic substance in the walls of 

 cells as, for instance, in the Grasses and the Equisetacese, and 

 the Cane-Palms (Calamus), where the epidermal structures are so 

 loaded with silex that they not only acquire a hard texture, render- 

 ing them harsh to the touch, but, when the organic matter is de- 

 stroyed by burning, a complete skeleton of the tissue remains, 

 entirely formed of silex. The siliceous coats of the Diatomece 

 afford another striking example. 



It is not yet clearly made out whether the silex is here deposited in a 

 layer upon the cell-membrane, or interpenetrates its substance ; but the 

 latter is probably the real state of the case. The pericarp of some plants, 

 as Lithospermum, contains lime, in what form it is not certain ; but the 

 calcium carbonate incrustin^ the cells of many species of Chara is clearly 

 a mechanical deposit upon the outside of the membrane. 



The membranous wall of the vegetable cell is for the most part 

 a permanent structure : forming the " skeleton " of plants, ti 

 usually remains entire until the decay or destruction of the 

 organism in which it exists. It is, as previously stated, in some 

 cases subjected to periodical variations in thickness, accord- 

 ing to season. Frequently it becomes absorbed or dissolved, 



