CHEMICAL REACTIONS OF PLANT TISSUES 61 



properties are briefly noted under the following head- 

 ings. 



Starch. The empirical formula assigned to starch 

 is (CeHioOs)*; but there are differences of opinion 

 as to whether the starch grain is homogenous in struc- 

 ture and whether it is a simple substance or a mix- 

 ture. Starch is the most widely distributed of all 

 the materials classed as cell contents, and in amount 

 exceeds all others. Starch grains swell, upon the 

 addition of boiling water, and form a pasty mass. 

 Complete solution of the grains is not effected unless 

 they are treated with superheated steam. Solutions 

 of alkali hydroxides even at moderate temperatures 

 cause formation of a paste with swelling of the grains. 

 Roasting transforms starch to dextrins which are 

 more or less soluble in water. The following reaction 

 is used in the identification of starch: 



1. Addition of dilute aqueous solution iodine-potassium iodide 

 colors starch grains blue. Heating causes a disappear- 

 ance of the color, but it reappears upon cooling. Very 

 characteristic results may be obtained by the addition 

 of chloral hydrate solution to specimens previously treated 

 with the iodine solution. The chloral acts as a clearing 

 agent, destroys many of the cell contents and finally 

 causes swelling and disintegration of the starch grains. 



Inulin. Inulin has the same empirical formula 

 as starch and occurs in solution form in plant tissues. 

 It can be precipitated in plant cells by the addition 

 of alcohol, but dissolves readily upon the subsequent 

 addition of water. The following reaction may be 

 used in the identification of inulin: 



1. Addition of aqueous iodine-potassium iodide solution colors 



inulin deposits brown. 



2. Pyrogallol solution colors inulin violet red. 



3. Acetic acid dissolves inulin, producing a greenish solution. 



