25.] CHAPTER I. THE CELL. 109 



fined to special cells, and frequently to special plants : starch- 

 grains ; fats ; profceid grains and crystalloids ; mineral crystals ; 

 the cell-sap, and the substances dissolved in it. 



a. Starch-grains are small solid granules of various shape 

 rounded, oval, lenticular, etc. consisting of starch with a certain 

 amount of water and a small proportion of incombustible ash. 

 They are specially abundant in those parts of plants which serve 

 as depositories of reserve-materials, e.g. rhizomes and roots of 

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

 such as those of the cereal and leguminous plants. They can be 

 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 ari 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. 98), the general 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 exposed 

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

 usually formed in the interior of the plastid (see Fig. 52) ; in the 

 latter case, on its surface. In both cases the mode of develop- 

 ment is the same ; a small rounded mass, the hilum*, which is the 

 organic centre of the grain, is first formed, and then the starchy 

 matter is deposited upon this in successive layers by the activity 

 of the plastid. If all parts of the primitive starch-grain are 

 equally within reach of the plastid, and if then the deposition 

 of new layers is equally active all over the circumference, the 

 grain maintains its rounded form, the hilum is its geometric as 

 well as its organic centre, and the layers are concentric. This, 

 from the nature of the case, most commonly occurs when the 

 grains are formed in the interior of the plastids. Very commonly, 



