CHEMISTRY OF VEGETABLES. 69 



it is recognised by the addition of iodine, when a bhie colour 

 is produced, owing to the formation of a blue iodide of 

 starch. Starch, as such, is not sokible in the fluids of the 

 body, but it is readily rendered soluble by the action of cer- 

 tain bodies of the nature of ferments. Starch is also ren- 

 dered soluble by the action of prolonged heat, or dilute 

 sulphuric acid, when it is converted into the gummy sub- 

 stance known as " Dextrine " or " British Gum." 



Cellulose is largely present in plants, and enters to a great 

 extent into the composition of the cells and vessels of all 

 vegetable tissues. Though allied to starch, it differs from 

 it in some important respects, especially in the fact that it 

 gives no blue colour on the addition of iodine. When 

 cellulose, however, is digested for a short time in sulphuric 

 acid, it is partially converted into starch, as shown by the 

 fact that iodine will then produce the fine blue colour of the 

 iodide of starch. The woody tissue which is deposited in 

 the hard parts of plants, and which is often called " Lignine," 

 may be regarded as probably a modification of cellulose. 



Si(gar is present in almost all plants, chiefly in their sap. 

 The two most important varieties of vegetable sugar are 

 *' cane-sugar" and "grape-sugar," both of which are capable 

 of crystallising. 



The nitrogeiiotis compounds of plants need little more 

 than mention, as they do not appear to differ in any essen- 

 tial respect from the albuminous compounds of animals. 

 The most important is "gluten," which occurs abundantly 

 in the seeds of Cereals and in the juices of many plants. 

 It is nearly allied to the "fibrine" of animals, and has the 

 power of spontaneously coagulating from its solutions. The 

 juices of many plants also contain a proteine compound 

 which is coagulated by heat, and which appears to be iden- 

 tical with the albumen of animals. Lastly, in the seeds of 

 peas, beans, and other legiiminous plants, there is found a 

 substance which is termed " legumine," and which appears 

 to be nearly allied to the ca*eine of milk. 



