116 H. VON MOHL ON CELLULOSE. 



split into two lamellae remaining attached to the two adjacent 

 cells, but the membrane situated between two cells remained un- 

 divided, separating from both cells, or remaining attached to one 

 of them, which of course must have been accompanied by a dis- 

 ruption of it in other places. When exactly the right period of 

 the action of the acid in which the wood is macerated, has been 

 hit, comparatively large pieces of the outer coat may be often 

 obtained isolated, by tearing up a piece of such wood with a 

 needle, since the cells may be readily extracted, as from a shell, 

 from the chambers formed by the cavities of the outer coat. 



Proceeding to the Vessels, the different layers of the walls of 

 the forms containing spiral or annular fibres behave in the reverse 

 way to those of cells. In the latter, namely, when they are 

 greatly lignified, the outer layers are usually saturated most 

 strongly with foreign compounds, and therefore offer the greatest 

 resistance to the action of sulphuric acid, while the inner layers, 

 as the youngest membranes, are frequently coloured a beautiful 

 blue by iodine and sulphuric acid ; in the vessels, on the con- 

 trary, the secondary structures (the fibres) are those which most 

 strongly resist sulphuric acid, and only acquire a yellow or at 

 most a green colour, while the tube on the inner wall of which 

 the fibres are deposited may acquire a bright blue with these 

 reagents. This difference is seen very beautifully in the vessel- 

 like elementary organs, furnished with flat, band-like spiral fibres, 

 of the wood of many Cactaceae, especially of the Mammillarice. 

 When these elementary organs are treated with boiling nitric 

 acid, both the fibre and the outer coat are coloured bright blue. 

 In like manner the spiral fibres of the vessels of herbaceous 

 plants, for instance of Asparagus, may be coloured bright blue 

 after treatment with nitric acid ; in the vessels of many plants, 

 however, especially in the spiral vessels of Sambucus nigra and 

 the scalariform vessels of Tree Ferns, a rather long-continued 

 boiling in the acid is requisite to destroy the green colour and 

 bring out the pure blue. 



The pitted vessels of the Dicotyledons approach the wood- 

 cells nearer than the spiral vessels in their behaviour with iodine, 

 since it is their outer layers which are principally infiltrated with 

 the foreign compounds coloured yellow by iodine. But treatment 

 with boiling nitric acid also produces the blue colour in all the 



