CARBON FOOD OF PLANTS. 63 



during the night, and then during the following day, the parts being cov- 

 ered from the light, no starch was formed in them. 



151. Starch in other parts of plants than the leaves. We 



may use the iodine test to search for starch in other parts of 

 plants than the leaves. If we cut a potato tuber, scrape some of 

 the cut surface into a pulp, and apply the iodine test, we obtain 

 a beautiful and distinct reaction showing the presence of starch. 

 Now we have learned that starch is only formed in the parts 

 containing chlorophyll. We have also learned that the starch 

 which has been formed in the leaves disappears from the leaf or 

 is transferred from the leaf. We judge therefore that the starch 

 which we have found in the tuber of the potato was formed first 

 in the green leaves of the plant, as a result of carbon assimila- 

 tion. From the leaves it is transferred in solution to the under- 

 ground stems, and stored in the tubers. The starch is stored 

 here by the plant to provide food for the growth of new plants 

 from the tubers, which are thus much more vigorous than the 

 plants would be if grown from the seed. 



152. The potato is only one example of a great many cases where starch 

 is stored up as a reserve material by plants, but not always in the form of 

 tubers. In the sweet potato and some other plants it is stored in the roots, 

 certain ones of the roots becoming very much thickened; in the onion it is 

 stored in certain leaves which form the onion bulb. 



153. Form of starch grains. Where starch is stored as a reserve material 

 it occurs in grains which usually have certain characters peculiar to the 

 species of plant in which they are found. They vary in size in many 

 different plants, and to some extent in form also. If we scrape some of the 

 cut surface of the potato tuber into a pulp and mount a small quantity in 

 water, or make a thin section for microscopic examination, we will find 

 large starch grains of a beautiful structure. The grains are oval in 

 form and more or less irregular in outline. But the striking peculiarity is 

 the presence of what seem to be alternating dark and light lines in the starch 

 grain. We note that the lines form irregular rings, which are smaller 

 and smaller until we come to the small central spot termed the "hilum " of 

 the starch grain. It is supposed that these apparent lines in the starch 

 grain are caused by the starch substance being deposited in alternating dense 

 and dilute layers, the dilute layers containing more water than the dense 

 ones; others think that the successive layers from the hilum outward are 



