24] CHAPTER I. THE CELL. 79 



as depositories of reserve-materials, e.g. rhizomes, jmd roots_of 

 perennial plants during the winter, tubers of the potato, seeds . 

 such as those of the cereal and leguminous plants. They canjbe 

 extracted by maceration from the organs in which they occur, and 

 then appear as a white powder which is known as' starch. Starch 

 is a carbohydrate ; its percentage composition is the same as that 

 of cellulose, and may be represented as C 6 H 10 5 , but its molecule 

 is smaller and less complex. It is readily detected by the cha- 

 racteristic blue colour which it assumes on treatment with an 

 aqueous solution of iodine. When boiled with water, or when 

 treated with potash, the grains swell enormously and form a paste. 



The substance of the starch-grain is always stratified, being 

 disposed in layers round an organic centre, the hilum + this stra- 

 tification, as also in the case of cell-walls, is the result of the 

 deposition of successive layers one on the other. The hilum is 

 the most watery portion of the grain, whilst the external layer is 

 the most dense. 



It is, as already mentioned (p. 70), the rule that starch-- 

 grains are produced by means of plastids : in parts of plants ex- 

 posed to light, by chloroplastids ; in parts of plants not__ex|)osed 

 to light, by leucoplastids. In the former case the_ grains are 

 usually formed in the interior of the plastid (see Fig. 40) ; in the 

 latter case, on its surface (Fig. 39). 



It not uncommonly happens th&t^compound starch-grains are to 

 be found. Sjmn'ouxly compound grains are simply grains which 

 have become adherent in consequence of mutual pressure : they 

 occur frequently in the interior of the plastids (see Fig. 40). The 

 truly compound grains (Fig. 53 B E) are formed in this way, 

 that one plastid produces simultaneously two or more rudimentary 

 starch-grains ; as these increase in size, they eventually come into 

 contact ; the further deposition of starchy layers must necessarily 

 be of such a kind that they surround, not each individual grain, 

 but the aggregate of adjacent grains ; the young grains thus be- 

 come bound together by investing layers, and a grain is produced 

 which has apparently a number of hila. 



The form of the starch-grains is characteristic in the different 

 plants in which they occur ; thus those of the Potato (Fig. 53) are 

 excentrically oval ; those of leguminous plants (Fig. 55), concen- 

 trically oval ; those of Rye, Wheat, and Barley, lenticular (Fig. 

 56). 



The distribution of starch throughout the different classes of 



