H. VON MOHL ON CELLULOSE. 105 



cause any change in the texture of the cells. The blue colour 

 shows itself uniformly, whether we investigate thin-walled, still 

 succulent cells, such as the rind-cells of woody plants or herba- 

 ceous vegetables, the cells of the parenchyma of the leaf and 

 petiole, or the dead cells of the pith or medullary rays of old 

 wood. The walls of epidermal cells saturated with cuticular 

 substance, and the cork and periderm of many plants, only, are 

 inaccessible to the action of nitric acid ; in this latter respect, 

 however, the cells of the periderm and cork of other plants form 

 an exception, since cellulose can be demonstrated in them not 

 only by potash but also by nitric acid, for instance in the peri- 

 derm of Plosslea, the cork of Sambucus nigra, Acer campestre, 

 Euonymus europceus, and Ulmus campestris. Yet in these cases 

 it is generally necessary to boil the preparation for a long time 

 in the acid, and the effect is mostly imperfect, as these parts do 

 not usually become coloured blue completely after this treat- 

 ment; there exist, however, some structures belonging to the 

 cork system in which nitric acid is capable of producing a perfect 

 blue colour, while only a greenish tint is obtained by the appli- 

 cation of caustic potash, for example the spines belonging to 

 the suberous system of Bombax and the corky rind of the rhizome 

 of Tamus Elephantipes. 



The cell-membranes which assume a blue colour after the 

 boiling with nitric acid usually form a very permanent combina- 

 tion with iodine. While in other cases the iodine which has com- 

 bined with a thin section of vegetable structure usually evapo- 

 rates entirely or in great part if the preparation is exposed for a 

 couple of days to the air, and may be extracted in a few seconds 

 with alcohol, preparations boiled with nitric acid and saturated 

 with iodine may often be left lying for weeks in the air without 

 the colour becoming perceptibly paler. In particular cases the 

 iodine combined with the membrane obstinately resisted not 

 only exposure to considerable heat^ but the action of almost 

 absolute alcohol heated to the boiling-point. By alkalies, on the 

 contrary, especially by caustic ammonia, the iodine may be very 

 quickly extracted from the membrane. Only the cells of a few 

 of the plants which I investigated, in particular those of the 

 petiole of Cycas revoluta, formed exceptions to this rule, that the 

 iodine combined very firmly with the membrane. 



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