STORAGE IN STEMS AND LEAVES 



79 



daylight usually gives no result. However, in case the leaf is 

 cut off from the stem before nightfall, it responds readily to the 

 iodine test for starch in the morning. This, of course, shows 

 that the starch made during the day had no outlet and there- 

 fore remained in the leaf cells where it was formed. Very 

 generally starch carried away from any part of the plant 

 body to another part is first changed 

 to sugar and travels in the form of a 

 very weak solution of sugar in water. 

 On its arrival at the storage region 

 (as in the case of the potato plant at 

 the tuber) the dissolved sugar is re- 

 converted into starch by the action of 

 minute colorless corpuscles of proto- 

 plasm known as leucoplasts. The starch 

 grains deposited for storage (Fig. 64) 

 are many times larger and show a far 

 more definite structure than those 

 formed in the chloroplasts during 

 photosynthesis. 



72. How plant food is carried; diffusion. If a little molasses 

 is poured into a straight-sided jar and water is carefully added, 

 a disk of porous paper being first laid on the surface of the 

 molasses to prevent instantaneous mixing, the water will for 

 a considerable time appear clear and colorless. Only after 

 some hours will the molasses rise and mingle much with the 

 water, or the latter perceptibly thin the molasses. This 

 process by which two liquids in contact become mixed by 

 the interchange of inconceivably minute portions (molecules) 

 of both liquids is called diffusion. The interchange of diffusi- 

 ble liquids through a membrane without visible pores, such as 

 an ordinary cell wall, is called osmosis. Ordinarily in osmosis 

 the stronger flow is from the less dense to the denser liquid. 

 In the case of the starch-loaded leaf (Sect. 71) it is evident 

 that, as fast as the starch grains temporarily deposited in 

 the chloroplasts are changed into sugar, some of the sugar 



FIG. 64. Starch from root- 

 stock of Canna. Magnified 

 300 diameters 



