208 Plant Physiology 



Starch then is an accumulation product apparently 

 conditioned only when the tension of the sugar which has 

 been produced is considerable, at least so considerable 

 that the cell is unable to use the surplus in building up the 

 permanent structures, or to remove it fast enough. Starch 

 is deposited within the chloroplasts in the form of small 

 granules. During the growing season it normally accu- 

 mulates in most leaves through the day, or so long as the 

 leaves are exposed to strong light ; while during the photo- 

 synthetic inactivity of the night much or all of this starch 

 may be removed. In most cases the leaf will be depleted 

 of starch if placed in the dark for a period of 12 hours, if 

 the leaf is not in itself a storage organ. The process of 

 starch removal and subsequent deposition, when that 

 occurs, invites special consideration later. 



In those plants forming starch abundantly in the leaves 

 it is often desirable, and extremely convenient, to employ 

 the relative accumulation of starch as a rough qualitative 

 indication of photosynthetic activity. Leaves from which 

 chlorophyll has been extracted may be stained with a 

 weak alcoholic solution (tincture) of iodine, the leaves 

 being preferably placed on a white plate to be stained. 

 When added to a weak suspension of starch, or to a weak 

 starch paste, iodine yields an intense blue or blue-black 

 color. Starch in the leaf, or in other tissues, is, however, 

 considerably obscured, and it often gives a blue-brown 

 or even brown-black reaction. Plants of the iris, lily, 

 amaryllis, and orchid families form, as a rule, little or no 

 starch. 



113. The diffusion process. - - It has already been 

 shown that the leaf (or an analogous structure) is an ad- 



