THE VITAL PROPERTIES OF THE CELL 



163 



Fig. 69. Starch grains from a Potato tuber 

 (after Strasburger, Pmct. Bot., Fig. 3): A simple 

 grain ; B semi-compound grain ; C and D com- 

 pound grains; c the hilum. (x540.) 



points, whilst at others they may be as large as 2 mm. in circum- 

 ference. Their reaction towards iodine solution is characteristic ; 

 they become either dark or 



" A 

 c 



light blue according to the 

 strength of the solution. 

 In warm water they swell 

 up considerably, and if fur- 

 ther heated turn into a 

 paste. 



Their shape also varies, 

 being sometimes oval, some- 

 times round, and sometimes 

 irregular. When strongly 

 magnified they are seen to 

 be distinctly stratified, and 

 in an optical section bright- 

 broad bands are seen to 

 alternate with more narrow 

 dark ones. JNageli explains 

 this appearance by the sup- 

 position that the starch 

 grain is composed of lamella? of starch, substance, which are alter- 

 nately rich and poor in water. Strasburger (V. 31), on the other 

 hand, is of opinion, that " the darker lines represent the specially 

 marked adhesion surfaces of consecutive lamella?, which," he con- 

 siders, "are more or less identical with each other in composition." 



The lamella? (Fig. 69) are arranged round a hilum, which is 

 either situated in the centre of the whole grain (J?, C) or, as is 

 more frequently the case, is eccentric in position (A). Further it 

 is not rare to find starch grains, which consist of two (B, 0) or 

 three (D) systems of lamella?, united together; these are termed 

 compound grains, in contradistinction to others which contain one 

 single hilum. AVhen the hilum is in the centre, the strata of starch 

 surrounding it are fairly uniform in thickness. On the other 

 hand when its position is eccentric, only the inner layers surround 

 it completely, whilst the peripheral layers are of greatest thick- 

 ness on that side which is turned away from the hilum, and grow 

 thinner and thinner as they approach it, becoming finally so 

 narrow, that they either fuse with neighbouring lamella?, or end 

 freely. 



In each starch grain the amount of water contained is greatest 



