SECOND SECTION. 

 The Molecular Forces in Plants. 



I. THE MOST IMPORTANT ORGANISED STRUCTURES 

 OF PLANT CELLS. 



42. The Membranes of Plant Cells. 



THE membranes of plant cells are by no means always com- 

 posed chiefly of cellulose, but are frequently more or less rich in 

 other substances, which we may call in general imbedded sub- 

 stances (Einlagerungskorper). To such deposition of foreign 

 substances is due, e.g., the cuticularisation and lignification of 

 membranes, with which we shall deal further in the sequel. In 

 many other cases, however, cellulose does form the most essential 

 constituent of the cell membranes, as is immediately shown by 

 their behaviour towards various reagents. 



We place on a slide in a, drop of water hairs from the seed of a 

 Gossypium or a few fibres of cotton wool. It is readily deter- 

 mined that the hairs, which in general are conical in form, have 

 comparatively thick membranes, and only stain brownish on an 

 addition of iodised solution of Potassium iodide (prepared by 

 dissolving 0'05 gr. of Iodine and O2 gr. of Potassium iodide in 

 15 gr. of water). We now run in Sulphuric acid from the edge of 

 the cover- glass (a mixture of 2 parts of concentrated Sulphuric 

 acid and 1 part of water), and at once observe that the hairs 

 stain blue. The membranes of other cells which, like those of 

 cotton hairs, consist chiefly of cellulose, give the same reaction. 

 Similarly all cell membranes which consist essentially of cellulose, 

 stain violet on treatment with iodised solution of Zinc chloride, 

 as we may easily ascertain by treating a few fibres of cotton wool 

 on the slide with the reagent. Chlor-zinc-iod. solution is prepared 

 as follows : We dissolve pure rod zinc in pure Hydrochloric acid, 

 at the ordinary temperature, until saturated, evaporate in presence 

 of excess of metallic zinc to the consistency of Sulphuric acid, add 



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