THE SKELETON OF THE PLANT 45 



Under certain conditions it is capable of imbibing con- 

 siderable quantities of water, and in consequence swelling 

 to a greater or less extent. Under ordinary conditions it is 

 freely permeable by water. It is composed of a substance 

 commonly termed cellulose, whose chemical composition is 

 represented by the formula n(C 6 H 10 5 ), the value of n not 

 having yet been accurately determined. This substance is 

 related to such bodies as starch, sugar, &c., being a mem- 

 ber 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 circumstances it can yield nutri- 

 tive material for the use of the plant. Cellulose possesses 

 the peculiar property of becoming a deep blue in colour 

 when treated with iodine in the presence of sulphuric acid, 

 chloride of zinc, 'or other hydrating 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 phos- 

 phoric, cause it to imbibe water and swell up, ultimately 

 becoming gelatinous and dissolving. Certain soluble fer- 

 ments affect it similarly. 



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

 found to be doubly refractive. 



It would probably be more accurate to speak of a group 

 of celluloses, for several varieties are known to occur in 

 different parts of plants. 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 Lycopodium, the endosperm of the Pseony, the 

 cotyledons of some of the Leguminosse, &c. The walls of 

 the hyphaB of the fungi are peculiar, in that they will not 

 give the blue colour with iodine even after treatment with 

 hydrating reagents. Kecent observations suggest that this 

 variety ot cell-wall approaches in composition the chitin of 

 the animal kingdom. 



The celluloses which have been so far examined have 

 been divided into three categories, according to the ease 



