i6 



GROWTH AND WORK OF PLANTS 



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

 tween reserve starch and transitory starch. Reserve starch is 

 that which is stored in special receptacles for future use and is 

 that which is utilized by man for commercial purposes. Transi- 

 tory starch is that which is formed during the day, and at night 

 is transported to the reserve organs. Reserve starch grains are 

 usually much larger than transitory starch grains. 



Fig. 24. 



A . Photomicrograph of starch grainy B. Photomicrograph of starch grains 



from potato. from potato, polarized light. 



22. 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 excen- 

 tric 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, 



