220 VEGETABLE PHYSIOLOGY 
Part of the sugar consequently gives rise to numerous 
minute grains of starch, which the plastid forms within itself 
and deposits in its own substance. This formation of a tem- 
porary store of starch not only relieves the over-saturation 
of the sap in the cell, but supplies the need of the protoplasm 
when the formation of sugar from carbon dioxide and water 
is interrupted by the failure of the daylight, being then re- 
converted into sugar. These minute granules are of very 
small dimensions, three or four of them being formed within 
each plastid. They have no apparent structure, but can be 
detected by treating the cell with a solution of iodine, 
which stains them blue. If a chloroplast so treated is 
examined with a high power of the micro- 
scope, it presents the appearance of fig. 104, 
the little grains of starch lying as blue specks 
in the.green substance. They can be seen 
Fic. 104.—Sranca inore distinctly if the leaf under examination 
ans ox ig bleached by warming it in alcohol, which 
nortasts.x260. dissolves out the chlorophyll. A leaf so 
treated turns blue wherever the light has had 
access to it, not only showing the formation of the starch, 
but allowing its exact locality to be determined with 
absolute precision. In fact, this test may be applied 
to ascertain whether the chlorophyll apparatus of a 
part is at any time active, the deposition of the starch 
taking place within a few minutes of the commencement 
of carbohydrate construction. This rapidity of appear- 
ance led indeed to the old view that the construction of 
starch rather than sugar was the immediate object of the 
chlorophyll apparatus. The reasons we have given lead us 
preferably to the view that the starch is the expression of the 
superabundant supply, requiring that a certain portion shall 
be deposited in an insoluble form as a temporary reserve 
material, to allow the process of carbohydrate construction 
to proceed without intermission so long as the conditions 
are favourable. At the same time we cannot but notice 
that the appearance of starch in the chloroplasts is so rapid 
