74 PHYSIOLOG y. 



153. 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 photosynthesis. 

 From the leaves it is transferred in solution to the underground 

 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. 



154. 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 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 

 regularly of diminishing density, and that this gives the appearance of alter- 

 nating lines. The starch formed by plants is one of the organic substances 

 which are manufactured by plants, and it (or glucose) is the basis for the 

 formation of other organic substances in the plant. Without such organic 

 substances green plants cannot make any appreciable increase of plant 

 substance, though a considerable increase in size of the plant may take 

 place. ' 



NOTE. The organic compounds resulting from photosynthesis, since 

 they are formed by the union of carbon, hydrogen, and oxygen in such a 

 way that the hydrogen and oxygen are usually present in the same proper- 



