112 H. VON MOHL ON CELLULOSE. 



by blue, although not brightly coloured, membrane, as I have 

 seen most distinctly in the wood of Taxus baccata and Abies 

 pectinata. But I must expressly remark, that observation of 

 this, as well as of the membrane which closes the pits of the par- 

 enchyma-cells, requires a microscope of the highest quality, fur- 

 nished with very strong objectives ; with objectives not of very 

 short focus, i. e. unless at least less than a line, even when the 

 image they give is perfectly free from error, the membrane closing 

 the pits will be sought in vain, since the penetrating power of 

 the microscope will be too small. 



The most difficult point in the investigation of the wood-cells 

 is that of their outermost membrane (Mulder's " outer cell-coat/' 

 Harting^s "cuticle of the wood-cells"). 



In the first place, the remark perhaps may not be superfluous 

 in reference to this membrane, that in many wood-cells we meet 

 with a case similar to that in the above-described pith-ceils of 

 'Clematis and the liber of Calamus and Cocos botryophora, 

 namely, that when a cross-section of the cell is soaked with 

 iodine, layers of two kinds may be distinguished, a thick inner 

 one, very brightly coloured, and a thinner outer one which 

 acquires a darker yellow colour with iodine and might readily be 

 taken for the primary membrane of the cell, e. g. in Buxus, in 

 particular cells of the wood of Erythrina caffra and of many 

 kinds of Ficus. This outer layer withstands the action of sulphuric 

 acid much more strongly than the inner, so that a weaker acid 

 suffices to cause a strong swelling up of the inner layer, bringing 

 out a blue or green colour, while the outer layer is quite unaf- 

 fected and remains yellowish brown. A stronger acid, however, 

 is capable of producing a green colour in the outer layer, or at 

 least of bleaching and dissolving it when the action is allowed to 

 continue for some time. Treatment of a cross section with boiling 

 nitric acid leaves no doubt of the true nature of the conditions, 

 since in a preparation so treated both layers are coloured bright 

 blue by iodine, and dilute sulphuric acid quickly dissolves even 

 the outer layer, so that any confusion of this with the outer cell- 

 membrane is out of the question. 



The latter exhibits the same qualities as I have described 

 above of the outer coat of the parenchyma and liber; it is 

 extremely thin, withstands the action of sulphuric acid, and 



