188 HISTOLOGY OF MEDICINAL PLANTS 



In central hilum starch grains the grain is laid down around 

 the hilum in the form of concentric layers. Thes$ layers are 

 of variable density. The dense layers are formed when plenty 

 of sugar is available, and the less dense layers are formed when 

 little sugar is available. The unequal density of the different 

 layers gives the striated appearance characteristic of so many 

 starch grains. 



In eccentric hilum starch grains the starch will be deposited 

 in layers which are outside of and successively farther from the 

 hilum. 



The term hilum has come to have a broader meaning than 

 formerly. Hilum includes at the present time not only the 

 starting-point of the starch grain, but the fissures which form 

 in the grain upon drying. In all cases these fissures originate 

 in the starting-point, hilum, and in some cases extend for some 

 distance from it. The hilum, when excentral, may occur in 

 the broad end of the grain, galanga, and geranium (Plate 71, 

 Figs, i and 3), or in the narrow end of the grain, zedoary (Plate 

 71, Fig. 4). 



NATURE OF THE HILUM 



The hilum, whether central or excentral, may be rounded 

 (Plate 75, Fig. i); or simple cleft, which may be straight (Plate 

 71, Fig. i); or curved cleft (Plate 71, Fig. 2); or the hilum may 

 be a multiple cleft (Plate 74, Fig. 3). 



In studying starches use cold water as the mounting medium, 

 because in cold water the form and structure are best shown, 

 and because there is no chemical action on the starch. On the 

 other hand, the form and structure will vary considerably if 

 the starch is mounted in hot water or in solutions of alkalies 

 or acids. The hilum appears colorless when in sharp focus, and 

 black when out of focus. 



Starch grains, when boiled with water, swell up and finally 

 disintegrate to form starch paste. 



Starch paste turns blue upon the addition of a few drops of 

 weak lugol solution. Upon heating, this blue solution is de- 

 colorized, but the color reappears upon cooling. If a strong 

 solution of lugol is used in testing, the color will be bluish black. 



