40 VEGETABLE PHYSIOLOGY 



represented by the formula ^(C G H, 5 ), the value of n 

 not having yet been accurately determined. This sub- 

 stance is related to such bodies as starch, sugar, &c., 

 being a member of the group of carbohydrates. It is 

 capable, under the action of hydrating reagents, of being 

 converted into a form of sugar, and under certain circum- 

 stances it can yield nutritive material for the use of the 

 plant. Cellulose possesses the peculiar property of becom- 

 ing a deep blue in colour when treated with iodine in the 

 presence of sulphuric acid, chloride of zinc, or other hydrat- 

 ing reagent. It dissolves with readiness in a solution of 

 ammonio-cupric sulphate (Schweizer's reagent), but is not 

 soluble in dilute acids or alkalies. Strong mineral acids, 

 such as sulphuric or phosphoric, cause it to imbibe water 

 and swell up, ultimately becoming gelatinous and dissolv- 

 ing. Certain soluble ferments affect it similarly. 



When the cell-wall is examined by polarised light it is 

 found to be doubly refractive. 



When cellulose is oxidised with strong nitric acid it 

 yields oxalic acid. 



Cellulose is capable of existing in more than one con- 

 dition. We find some kinds of it which will stain blue with 

 iodine without previous hydration. Examples of this 

 variety are found in the cell-walls of the bast of Lycopo- 

 dium, the endosperm of the Pseony. the cotyledons of some 

 of the Leguminosse, &c. The walls of the hyphae of the 

 fungi differ again, in that they will not give the blue colour 

 with iodine even a^fter treatment with hydrating reagents. 

 Eecent observations suggest that this variety of cell-wall ap- 

 proaches in composition the chitin'ot the animal kingdom. 



The celluloses which have been so far examined have 

 been divided into three categories, according to the ease with 

 which they can be made to undergo hydrolysis, and to yield 

 some variety of sugar by such treatment. The celluloses of 

 cotton fibres are perhaps the most resistent of all, and may 

 .be taken as representatives of the most refractory group. 

 The cellulose found in the main mass of the fundamental 



