ELEMENTARY STRUCTURE. 



21 



Fig. 38. 



plants, and directly exposed to light. "When starch occurs in 

 the active vegetating parts, it is then commonly invested by 

 chlorophyll granules. 



Starch is not only widely distributed through the different parts 

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

 classes of plants, except 

 the Fungi. Arrow-root 

 (Jig. 37), tapioca, sago 

 (^Jig. 38), tous-les-mois 

 (fig. 39), and potato 

 starch {fig. 40) may be 

 mentioned as familiar 

 examples of starches 

 derived from different 

 plants. In all cases 

 starch is a transitory 

 product stored up for 

 future use, resembling 



in this respect the fat ^9- S*"- West-India Arrow-rqot.-^FtV/. 



of animals. When thus 



meal. Both magnified 250 diameters. 



Sago 



required for the nutrition of the plant, it is converted previ- 

 ously, as will be afterAvards seen, into dextrine and sugar, 

 which are soluble -p- 



substances, and can 

 be at once applied to 

 the purposes of nu- 

 trition, which is not 

 the case with starch 

 in its unaltei-ed con- 

 dition, as that is in- 

 soluble. 



Starch is composed 

 chemically of carbon 

 and the elements of 

 water. Its exact com- 

 position, according 

 to Payen, being Cj^ 



39, 



Fig. 40. 



/ 



/ 



Fig. 39. 



Tous-les-mois Fig. 40. Potato starch. 



Both magnified 2.50 diameters. 



Hio Oio ; the same as cellulose, with which it is therefore 

 isomeric. Starch, however, never occurs naturally in a perfectly 

 pure condition, but it always contains 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 other purposes. Starch 

 is insoluble in cold water, alcohol, ether, or oils. By the action 

 of boiling Avater it SAvells up and forms a gelatinous mass. 

 Iodine when apphed to it gives a blue colour or some shade of 

 c 3 



