CONSTITUENTS OF PLANTS 373 



animals. From 0.6 to 07 per cent, is found in the muscles, but 

 it disappears while the animal is at hard work or starving. It is 

 contained in quantity in the liver, and is the reserve material 

 formed from the excess of carbohydrates for the furnishing of 

 sugar to the blood. 



Gums. Gums are found in many vegetable materials. They 

 often exude from cut places. Both hexoses and pentoses are 

 formed by fHe hydrolysis of gums. Some of the gums are the 

 best materials for the preparation of the pentose sugars. 



The following are some gums and the sugars they yield on 

 hydrolysis : 



Gum arable yields galactose and arabinose. 



Wood gum, extracted from wood by alkalies. 



Cherry gum yields arabinose. 



Peach gum yields galactose and arabinose. 



Barley gum yields galactose and xylose. 



Galactan yields galactose and other sugars, and is found in 

 leguminous plants. 



Pectins. These are substances found in fruits and some fleshy 

 roots. They are soluble in water, and precipitated as a jelly-like 

 mass by alcohol. When boiled sufficiently, they jelly on cooling. 

 If boiled too long, they will not jell. Pectins appear to be closely 

 related to the carbohydrates, or belong to them. They are found 

 in apples, pears, quince, cranberries, beets, turnips, etc. 



Cellulose, C 6 H 10 O 5 . Cellulose is the chief constituent of the 

 cell walls of plants. It is insoluble in warm dilute acids or 

 alkalies. In young parts of plants, the cell walls are composed 

 of almost pure cellulose. In older organs the cellulose is inter- 

 penetrated with "incrusting material." Cellulose is found 

 abundantly in wood tissue and woody tissue of all kinds. Cotton 

 is almost pure cellulose. Flax and hemp are composed largely of 

 cellulose. It may be prepared by treating the material successively 

 with ether, boiling dilute acid, boiling dilute alkali, and then with 

 cold dilute nitric acid and potassium chlorate to remove the in- 

 crusting material. The residue consists of cellulose. Cellulose is 

 a colorless insoluble material. Its molecular weight is unknown 



