M. Mohl on the Growth of Cell- Membrane. 2G7 



differences which distinguish the secondary cell-membranes from 

 true cellulose will appear to us of less consequence. This proof 

 however, in my opinion, the outer wood-membrane furnishes 

 the most clearly of all. I have already, reasoning on the exami- 

 nation of the wood of Pinus sylvesiris, on anatomical grounds, 

 shown the outer membrane to be the primary ; chemical exami- 

 nation of young cells does not contradict this, since at the time 

 when the borders of the dots are already perfectly formed, but 

 neither the dots themselves nor the inner layers in which they 

 are situated yet exist, the membrane of these cells is coloured 

 by iodine and sulphuric acid, not yellow, but blue. The relation 

 of the membrane to the borders of the dots leaves no doubt that 

 we have here to do with the same membrane which subsequently 

 appears as the outer layer of wood with wholly altered chemical 

 properties. We must therefore assume, that the cellulose of 

 which this membrane originally consists is either absorbed and 

 replaced by the substance of the outer ligneous layer, or that 

 the latter penetrates into the cellulose and prevents its reaction 

 towards iodine and sulphuric acid. Which of these cases occurs, 

 cannot be decided until some solvent for the substance of this 

 membrane shall be found which will not at the same time dis- 

 solve the cellulose, yet remaining in it, or at least will give some 

 evidence of its presence. Since such a solvent is not yet known, 

 the question must for the present remain open ; perhaps the fol- 

 lowing observations may afford a hint. 



I tried next whether the action of stronger sulphuric acid on 

 the outermost layer of membrane of the wood-cell, especially in 

 the Conifera, would produce a blue colour, but did not in this 

 way attain my object. The formation of the blue colour de- 

 pends therefore on the simultaneous reaction of sulphuric acid, 

 iodine, water and cellulose. If concentrated sulphuric acid be 

 applied, the cell-membranes do not become blue so long as the 

 necessary water is wanting ; or if they be already coloured blue, 

 this colour is soon lost again, and the secondary layers become 

 dissolved. This solution however affords no convenient means 

 by which to obtain the outer membrane isolated, and to examine 

 the colour which, after the action of a stronger acid, it assumes 

 with iodine and weak acid, since so soon as water and tincture of 

 iodine are added to the fluid in which the preparation lies, the 

 dissolved cellulose is precipitated again of a very dark blue co- 

 lour, and envelopes the outer membranes in such a manner, that 

 no certain conclusions can be drawn as to its colour. I sought 

 therefore to separate the outer membrane from the secondary 

 layers before I applied the sulphuric acid to it. This may always 

 be done in the fibres of the liber of the black fibrous wood of the 

 Palm which is imported from Brazil for the manufacture of sticks. 



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