FOODS IN PLANTS 41 



of a weak solution of sugar in water. On its arrival at the 

 storage region (for example, the tuber of the potato plant) 

 the dissolved sugar is reconverted into starch by the action 

 of minute, colorless corpuscles of protoplasm, known as leuco- 

 plasts. The starch grains deposited for storage (fig. 28) are 

 many times larger and show a far more definite structure 

 than those formed in the chloroplasts during photosynthesis. 

 40. Diffusion and osmosis. It is clear that plant food must 

 pass from within certain cells into others, and the way in which 

 this occurs may be made clearer by the use of simple illustra- 

 tions. If some dried raisins or primes are placed in pure water 

 it will soon be noted that the outside membranes, which at 

 first were contracted and wrinkled, have become distended. 

 Water has passed through the fruit coating and is retained 

 within it. After a longer time it will be found that the water 

 outside the fruit has become sweet, owing to the outward pas- 

 sage of dissolved sugar from within the fruit. If a little molasses 

 is poured into a straight-sided jar, and a disk of porous paper 

 is placed so as to cover the molasses (to prevent instantaneous 

 mixing), and water is then carefully poured upon the disk of 

 paper, 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 tendency 

 is for the two liquids to become completely intermingled, so 

 that at last all portions of the mixture are of precisely the 

 same composition. Similarly, if a dense liquid surrounds a 

 plant cell, water passes more rapidly outward than inward, and 

 the remaining internal parts of the cell collapse, or plasmolyze 

 (fig. 29), because of the loss of water. This merely illustrates 

 the fact that interchange of liquids may take place in either 

 direction through a membrane, but is more rapid from the 

 less dense to the more dense liquid. The mingling of liquids 

 that are separated by a partition which one or both of them 



