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 hy the use of simple illustra- 
tions. If some dried raisins or prunes 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 
