[Chap. XV SYNTHESIS OF STARCHES 131 



leaves from a plant with a knife and place the cut ends of the petioles 

 in beakers of water containing different concentrations of sugar and also 

 different kinds of sugar. They can then be readily exposed to different 

 environmental conditions. Even the leaves in which starch ordinarily 

 does not form may produce it under these experimental conditions either 

 in the light or in the dark. Best results are usually obtained with about a 

 10 per cent solution of sugar. Since starch is made from glucose, one 

 might expect to find starch in the leaves only when glucose is placed in 

 the beakers, unless he recalls that by the influence of the enzymes in 

 plant cells certain kinds of sugars may be transformed to others. In such 

 experiments starch is formed in the leaves whether the sugar in the 

 beakers is glucose, fructose, mannose, galactose, sucrose, or maltose. 

 Even certain alcohols may be converted to starch in the leaves of 

 plants. 



Starch accumulation. Sugar is involved directly or indirectly in all 

 the major physiological processes in plants. Starch, on the contrary, 

 merely accumulates. In time it may be digested back to glucose, either 

 by the plant that makes it, or by some other organism that eats it. Starch 

 is formed in plastids (leucoplasts and chloroplasts ) and occurs in the 

 cells as small grains that are insoluble in water; therefore it does not pass 

 from cell to cell. When starch is found in a cell one may infer that it was 

 made in that cell. It may be formed in all kinds of living plant tissues, 

 in either hght or darkness if sugar is present. The fact that it accumu- 

 lates sooner or often er in certain cells or tissues indicates that the condi- 

 tions affecting its formation are not uniform throughout the plant. There 

 may be a lack of general uniformity in the types of plastids, the amounts 

 and kinds of enzymes, the concentration of sugar, and the acidity of 

 the cells. 



This lack of uniformity among cells is easily demonstrated with green 

 and white variegated leaves. Since sugar is made only in the chloroplasts, 

 its concentration in the non-green parts of the leaves is comparatively 

 low, and starch is usuallv formed only in the chlorenchyma of these 

 leaves. This fact is demonstrated by the ordinary iodine test for starch 

 in leaves after the chlorophyll has been removed with hot alcohol. 



If these variegated leaves are first detached from the plant and placed 

 for about one day in a beaker containing a sugar solution as described 

 above, starch may then be found in the non-green parts of the leaf. 

 Variegated leaves of geranium are excellent materials for these tests 

 (Fig. 59). It has been reported that even with this special treatment 



