H. VON MOHL ON CELLULOSE. 9? 



this never causes iodine to produce a blue colour in the substance 

 of the membrane, but the latter is always coloured yellow or 

 brown by the application of those reagents. The results are very 

 different when cuticle is subjected for some time to the action of 

 caustic potash. For this purpose a thin section of some epidermis 

 possessing a thick cuticle, for example that of the leaf of Aloe 

 obliqua, must be kept from 24 to 48 hours in a strong solution 

 of caustic potash, between two slips of glass, at ordinary tempe- 

 ratures. The solution I used was so concentrated, that crystals 

 of hydrate of potash were formed when the temperature of the 

 room fell to freezing-point. It is not requisite that the potash 

 should be chemically pure. When the action of the potash on 

 the cuticle was strong enough, the microscope revealed that 

 numerous little drops exuded from it, of a tenacious fluid, not 

 mixing with the solution of potash, but becoming yellow with 

 iodine. The cuticle itself was somewhat swollen up and showed 

 itself (like the membrane of thick-walled cells treated with 

 sulphuric acid) to be composed of numerous superimposed 

 lamellae, which were not continued uninterruptedly from one cell 

 to another, and hence did not form a connected layer lying upon 

 the epidermis and distinguishable from it, but terminated at the 

 boundaries of contiguous epidermal cells and formed part of 

 their walls. In most cases the epidermal cells had expanded 

 somewhat in the direction of their breadth, and the segments of 

 the cuticle corresponding to the individual epidermal cells had 

 become more or less perfectly separated from each other. When 

 a few drops of a strong tincture of iodine* are applied to the 

 preparation, the latter dried, and then wetted with water, the 

 cuticle acquires as bright a blue colour as the walls of the epi- 

 dermal cells and the subjacent parenchyma. The purity of the 

 colour is increased, as in most cases in which a cellular membrane 

 is coloured blue by iodine without the application of sulphuric 

 acid, by allowing the preparation to dry up once or twice after 

 being saturated with iodine and wetted with water, and then 



* In these, as in all the following researches, I used a tincture of iodine, in 

 preparing which I added an excess of iodine, so that part of this remained 

 undissolved at the bottom of the alcoholic tincture. The attempt to apply a 

 tincture made with sulphuric aether instead of the alcoholic, so as to gain time 

 by the rapid drying, was not accompanied by good results, since this tincture 

 did not wet the preparation so perfectly as that made with alcohol. 



SCI EN. MEM.— Aa/. Hht. Voi,. I. Part II. 7 



