188 HISTOLOGY OF MEDICINAL PLANTS 
In central hilum starch grains the grain is laid down around 
the hilum in the form of concentric layers. These 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. 1 and 3), or in the narrow end of the grain, zedoary (Plate 
Vig Tig. 4). 
NATURE OF THE HILUM 
The hilum, whether central or excentral, may be rounded 
(Plate 75, Fig. 1); or simple cleft, which may be straight (Plate 
71, Fig. 1); 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. 
