

NATURE OF FOOD STORED IN SEEDS 



21 



plants. Commercial starch, while it will give the tests for 

 starch, is not suitable for the study of the grains, since they are 

 destroyed in its preparation. Starch becomes blue by treat- 

 ment with a few drops of a tincture of iodine, or, better, a solu- 

 tion of iodine in potassium iodide. Sometimes the color is more 

 or less of a purple or violet shade. We should distinguish be- 

 tween reserve starch and transitory starch. Reserve starch is 

 that which the plant stores in special receptacles for future use 

 and is that which is utilized by 

 man for commercial purposes. 

 Transitory starch is that which 

 the plant forms during the day, 

 and at night is transported to 

 the reserve organs. Reserve 

 starch grains are usually much 



Fig. 27. 

 Photomicrograph of starch grains from potato. 



larger than transitory starch 

 grains. 



29. Form and appearance 

 of starch grains. In the 

 potato tuber the starch grains 

 are packed in the cells. The 

 grains are of quite large size 

 as compared with many other 

 kinds of starch. They present 

 a very interesting and characteristic structure. The grain 

 appears to be stratified, the strata often being in excentric 

 layers. In potato starch these strata are usually in excentric 

 rings about a single spot called the hilum. These layers are 

 supposed to be made up of alternate dense and less dense layers 

 of the substance. Potato starch grains are oval to rhombic in 

 form. Starch grains of the corn are more or less angular and are 

 much smaller than those of the potato. Those of the bean and 

 other legumes are more or less kidney-shaped. In the corn and 

 bean starch grains there are often radiating lines from the hilum, 

 which appear like fissures. The starch grains of wheat are of two 

 kinds, very small ones, and quite large ones mixed in with them. 



