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(CgH,,0;), the value of nm not 
having yet been accurately determined. This substance is 
related to such bodies as starch, sugar, &c., bemg 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 Peony, the 
cotyledons of some of the Leguminose, &c. The walls of 
the hyphe of the fungi are peculiar, in that they wil] not 
give the blue colour with iodine even after treatment with 
hydrating reagents. Recent 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 
