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- 

 lUj 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 

 FIG. 104. STARCH more distinctly if the leaf under examination 

 BODIES OF N CO E is bleached by warming it in alcohol, which 

 KOPLASTS.X250. 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 



