74 PHYSIOLOGY. 



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, scrajie some of 

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

 a beautiful and distinct reaction showing the presen( e of starch. 

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

 containing ciiloropliyll. ^\'e have also learned that the starch 

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

 is transferred fom 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 photosvnthesis. 

 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 pro\'ide 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 reser\c material 

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

 species of plant in which they are found. Tliey vary in size in man\' 

 (different plants, and to some extent in fin-Tii also. If we scrape s^me of 

 the cut surface of the potato ttilici- int<") a I'ulp and nuiuiu a small quantity 

 in water, or make a thin section tor microscopic exaniiiiatiDU, we lind 

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

 form and more or less irregular in nutHiie. Btit 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 termcil 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 altern.iting dense 

 and dilute layers, the dilute layers containing more w.iter than the dense 

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

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

 nating lines. The starch formed by |ilants is one of tlie organic substances 

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

 formation of otlier organic subslam es in tlu' |ilant, ^\'ithoul sia h organic 

 substances green plants cannot make any appreciable increase uf plant 

 substance, though -x ( onsidiTalde int rease in si/e uf the plant nia>' take 

 place. 



NOTI':. — The organic compoimds resnlting from phoios\aitliesis, since 

 they are formed l)\' the union of carbon, liMlrogen, and oxygen in such a 

 way that the hydrogen and o.ween are usu.illv iiresenl in the same pronor- 



