214 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 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. 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 microscope, it presents the 
é; appearance of fig. 104, the little grains of 
starch lying as blue specks in the green 
substance. They can be seen more dis- 
Fic. 104.—Starcu —, ‘ . . , 
Graws i ran tinctly if the leaf under examination is 
aoe or ome, bleached by warming it in alcohol, which 
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 appearance 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 carbo- 
hydrate 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 when the conditions of carbo- 
