NONrNITROGENOUS ORGANIC CELL-CONSTITUENTS. 115 



of granulose in water rotates the plane of polarized light strongly to the 

 right. According to Payen, when starch is placed in a saturated solu- 

 tion of potassium iodide, or potassium bromide, it swells up to a stiff 

 jelly and increases twenty-five to thirty times in volume. This mass 

 .may then be dissolved in water, with only a slight residue of starch- 

 cellulose. Dilute acids will also dissolve granulose. 



The alteration of starch through the action of the diastatic ferments 

 will be described under the consideration of the action of the digestive 

 juices on the different food-stuffs. When starch is boiled with dilute 

 acids similar products result. 



When starch is subjected to dry heat at 150° to 100° C. it is gradu- 

 ally transformed into dextrin. When moisture, however, is present, quite 

 different compounds result, the starch being completely decomposed, with 

 the formation of carbon dioxide, formic acid, etc. In still higher tem- 

 peratures small quantities of brenzcatechin are formed, — a fact which is 

 of especial' interest, as it indicates the possibility of the conversion of 

 carbo-hydrates into members of the aromatic series. Oxalic acid results 

 from heating starch with nitric acid. 



The test for starch is iodine, which, with raw starch, or with starch- 

 mucilage, gives a deep-blue coloration which disappears on heating, to 

 return on cooling, if the heat has not been too prolonged. Starch is 

 also precipitated from its solutions bj r tannic acid in the form of a yellow, 

 flocculent sediment which is dissolved on heating. 



2. Cellulose (C 6 Hi O 6 ). — Cellulose forms the wall or cell-membrane 

 of vegetable cells, and may be regarded as the skeleton of plants. It is 

 formed by vegetable protoplasm out of other carbo-hydrates, such as 

 starch and sugar, and is capable of being again reconverted into other 

 members of the same group. It only very seldom occurs in a perfectly 

 pure condition. Young plants contain purer cellulose than older plants; 

 in the latter there is a greater percentage of ash. Cotton and Swedish 

 filter-paper are forms of comparatively pure cellulose. Cellulose is very 

 hygroscopic, but ammoniacal cupric oxide solution (Schneider's reagent) 

 is its only solvent. In sulphuric acid it first swells up and then dissolves 

 and forms a substance which is stained blue with iodine. This substance is 

 termed amyloid, but must not be confounded with the amyloid substance 

 of pathologists, which has been already described under the albuminous 

 bodies. Cellulose is also capable of being converted into the sugar 

 group by prolonged action of acids. Woody fibre is a modified form of 

 cellulose, which is due to the deposit within the cellulose of nitrogenous 

 substances ; it then has acquired a greater power of resistance to various 

 mechanical and chemical agents. In woody fibre cellulose has become 

 associated with a body richer in carbon and poorer in oxygen than cellu- 

 lose, and which is termed lignin ; its formula is C M TT M O u . The lignin 



