H. VON MOHL ON CELLULOSE. 113 



is coloured yellow by this and iodine. It is just as difficult with 

 this outer coat of the wood-cells as with the outer layer of the 

 parenchyma-cells, to answer the question whether it is coloured 

 yellow or blue by iodine, but similar reasons render it probable 

 here also that the latter is the case. For if a transverse section 

 of a Dicotyledonous wood which has been boiled in nitric acid 

 is soaked with iodine, a yellow membrane may be detected 

 between the blue-coloured cells ; if the action of the acid has 

 been more powerful, this yellow colour is more and more lost 

 and passes through green into a perfectly pure bright blue, so 

 that no trace of any yellow membrane lying between the blue 

 cells can be seen here any more than in the parenchyma-cells. 

 This is the behaviour, for example, of the wood of Abies pectinata, 

 Larioe europaa, Taxus baccata, Torreya tazifolia, Buxus semper- 

 virenSy Viburnum Lantana, Viscum album, Betula alba, Fayus 

 sylvatica, Clematis Vitalba, Erythrina caffra. When dilute 

 sulphuric acid is added, the cell-membranes are dissolved with 

 a slow bleaching, arid there remains behind a network of immea- 

 surably thin yellowish- brown pellicles, which correspond to the 

 boundaries of the cells. 



In most, perhaps in all woods, the intercellular passages 

 running between the wood-cells are filled up by an intercellular 

 substance, which is coloured yellow by iodine and sulphuric 

 acid, and not attacked by the latter, whence one might easily be 

 led to assume that this substance formed a common mass with 

 that membrane of the cells which likewise acquires a yellow 

 colour under these circumstances. But the incorrectness of 

 such a notion is proved by the examination of the preparations 

 boiled in nitric acid, since in these the intercellular substance 

 retains its yellow colour on the application of iodine, while the 

 outer cell- membrane is coloured blue. Whether the intercellular 

 substance of Dicotyledonous woods is wholly free from cellulose, 

 or contains it in a very strongly combined condition in which it 

 is not acted on by iodine, I cannot yet venture to decide. The 

 application of caustic potash to the investigation of this condi- 

 tion entirely failed me, and the application of nitric acid fur- 

 nished no decisive result. For if the boiling with nitric acid is 

 stopped before the texture of the cells is attacked, the intercel- 

 lular substance, as above mentioned, remains yellow on the 

 SCIEN. MEM. JVo/. Hist. VOL. I. PART II. 8 



