1 62 



BOTANY AND PHARMACOGNOSY. 



plants. The amount of starch present in the tissues of plants 

 varies. In the root of manihot as much as 70 per cent, has been 

 found. This constituent also varies in amount according to the 

 season of the year. Rosenberg has observed that in certain peren- 

 nial plants there is an increase in the amount of starch during 

 the winter months, whereas in other plants it decreases or may 

 entirely disappear during this period. In the latter case, from six 

 weeks to two months in the spring are required for its re-forma- 

 tion, and about an equal period is consumed in the fall in effect- 

 ing its solution. 



Fig. 95. Appearance of various starches under polarized light: I, II, III, potato- 

 starch grains; IV, wheat-starch grain; V, pea-starch grain. — After Dippel. 



Structure and Composition of Starch Grains. — The formula 

 which is generally accepted for starch is (C(.Hi(|03)n, this being 

 recognized by Pfeflfer, Tollens and Mylius. It is supposed that 

 the molecule of starch is quite complex, it being composed of dif- 

 ferent single groups of CoHijO^ or multiples of the same. While 

 this formula may be accepted in a general way, still it has been 

 shown that there are at least two substances which enter into the 

 composition of the starch grain, and more recent studies tend 



