46 THE LIFE OF THE PLANT 



whole series of reagents by which we can detect the 

 presence of albumen. Let us experiment with one of 

 these reagents, the most obvious if not the most certain. 

 I have in a glass a certain quantity of the white of an 

 egg in water. I add to it some ordinary syrup of sugar, 

 together with concentrated sulphuric acid. A pre- 

 cipitate forms which dissolves again, and all the liquid 

 gradually turns a splendid red colour. In this way 

 albuminoids can be detected by means of sulphuric acid 

 and sugar. 



There remains a third group, that of oils and fats. 

 We have no clear and simple reagents to produce in them 

 such characteristic changes of colour; but instead, as 

 we have already noticed, we have only to treat a sub- 

 stance in which the presence of oil or fat is suspected 

 with ether, and the ether will dissolve them. Then if 

 we expose this solution to the air, and let the ether 

 volatilise, we get oil or fat with its characteristic 

 properties. 



Now we can reproduce in cells under a microscope all 

 the reactions we have mentioned. Suppose we add 

 sugar and sulphuric acid to water in which a cell is being 

 observed. We shall notice the protoplasm turning 

 pink, which proves that it consists chiefly of albuminous 

 substances. Let us use Fehling's solution, and if the 

 cell-sap contains any trace of grape sugar we shall get 

 a red precipitate. We add a drop of iodine solution, and 

 notice that the small colourless grains in the cavity of 

 the cell turn blue : this indicates the presence of starch. 

 We take next iodine dissolved in zinc chloride solution, 

 and the whole cell-wall turns blue, which means that it 

 consists of cellulose. Finally we add ether, and notice 

 that the drops which had attracted our attention by 

 their oily appearance have disappeared, have dissolved, 

 which proves that they were drops of oil. Such is the 

 way that chemical analysis and microscopic investiga- 

 tion work hand in hand, mutually supplementing each 



