223 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 



D ^ appearance of fig. 108, the little grains of 



~ H starch lying as blue specks in the green 



substance. They can be seen more dis- 



FIG. 108. STARCH J ... 



GRAINS IN THE tinctly if the leaf under examination is 



BODIES OF CHLO- .. . , , , , . , 



ROPLASTS. x2so. 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- 



