CELL-CONTENTS AND FORMS OF CELLS. 143 



more or less distinct characteristics for the plant in which they 

 are found. Starch of this kind is usually spoken of as RESERVE 

 STARCH (Fig. 87). 



Occurrence of Starch. Starch is found in most of the algae 

 and many of the mosses, as well as in the ferns and higher plants. 

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

 In the grains of rice as much as 84.41 per cent, has been found. 

 This constituent also varies in amount according to the season 

 of the year. Rosenberg has observed that in certain perennial 

 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 



FIG. 89. A, starch grains of Iris floreriina showing peculiar horseshoe-like fissure 

 extending from point of origin of growth; B, irregular starch grains of calumba root; C, 

 peculiar beaked starch grains of ginger rhizome; D, starch grains of bean showing irregular 

 longitudinal fissures;. E, compound starch grains of oat. 



to two months in the spring are required for its re-formation, 

 and about an equal period is consumed in the fall in effecting its 

 solution. 



Structure and Composition of Starch Grains. The formula 

 which is generally accepted for starch is (C G H 10 O 5 ) n , this being 

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

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

 ferent single groups of C H 10 O- 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 

 to show that it is in the nature of a sphero-crystalloid, resembling 

 inulin in some respects. Starch grains have an interesting struc- 



