30 
HISTOLOGY OF VEGETABLES. 
Each granule has at one extremity a circular spot, 
termed the hilum, around which are a large number of 
curved lines, passing at first in circles concentrically, 
and subsequently in curves around the hilum, these 
markings are not, as has been imagined, the result of 
consecutive deposits, nor indications of increments of 
growth of the granule itself ; they are confined to the 
cell-wall, and are most probably mere tranverse pucker- 
ings or rugae in the membrane, of which, together with 
its amylaceous contents, the starch granule consists. If 
sulphuric acid, and heat he gradually applied to starch, 
the granules swell to three or four times their ordinary 
size, and the distended cell-walls lose all trace of the 
markings which previously existed. When starch is 
boiled in water, the cell-membrane swells and bursts ; the 
first stage of which is shown in Fig. 19, /, and the 
amylaceous matter becomes intimately mixed or dis- 
solved in the fluid. 
The presence of starch, however minute in quantity, 
can he immediately recognized by the addition of free 
iodine, which forms with it a compound of a beautiful 
blue colour, the iodide of starch — this test was discovered 
by Jurine. A little tincture of iodine added to the 
grains of starch in a section of the potato will make each 
granule blue, but will not so colour the vegetable cells 
in which these granules are stored up. If a small 
quantity of starch be boiled and poured into a vessel 
of cold water, the addition of a few drops of tincture 
of iodine will give the water a rich blue colour ; if a 
