ELEilENTARY STEUCTUEE. 



Fig. 56. 



parts of a plant, but it also occurs in varying quantity in all 

 classes of plants. Arrow- 

 root {fig. 53), sago, 

 {fi^. 54), tous-les-mois 

 {fig. 55), and potato 

 starch {fig. 56) may be 

 mentioned as familiar 

 examples of starches 

 derived from diflferent 

 plants. In all cases 

 starch is a transitory 

 product stored up for 

 future use, resembling 

 ^ in this respect the fat 



! of animals. When thus Fig. 53. West-India Arrow-root. Fig. 54. Sago 



\ required for the nutri- meal. Both magnified 250 diameters. 



\ tion of the plant, it is converted previously, as will be afterwards 

 \seen, into dextrin and sugar, which are soluble substances, and 

 -can be at once ap- 

 plied to the purposes 

 of nutrition, which 

 is not the case with 

 starch in its un- 

 altered condition, as 

 It is then insoluble, 

 f Starch is composed 

 chemically of carbon 

 and the elements of 

 water. Starch, how- 

 ever, never occurs 

 naturally in a per- 

 fectly pure condition, 

 but it always con- 

 tains a certain proportion of the peculiar secretions of the plant 

 from whence it is derived. These impurities can never under 

 ordinary circumstances be entirely removed, and hence arises the 

 difference in the value of the various starches used for food and 

 f other purposes. Starch is insoluble in cold water, alcohol, ether, 

 or oils. By the action of boiling water it swells up and forms 

 a gelatinous mass. Iodine when applied to it gives a blue 

 colour or some shade of violet, the distinguishing character of 

 starch and some other matters closely allied to it, as cellulose 

 and amyloid ; which latter is a substance found occasionally in 

 the secondary deposits in the albumen of some seeds. The blue 

 colour is at once destroyed by the application of heat and 

 alkalies. If starch be exposed to heat for a prolonged period,/ 

 it is converted into a soluble gummy substance, called dextrin-, 

 or British gum. A similar change is produced in starch by the ^ 



Fig. 55. Tousles-mois. Fig. 56. Potato starch. 



Both magnified 250 diameters. 



