The Planilet. 45 
a glass of clear water, we shall find that the lump will not 
retain its form and size, but infinitely small particles of it 
will become detached and move about to all parts of the 
water. This movement will not stop until the lump has 
entirely disappeared, and until every part of the water con- 
tains exactly as much of the aniline blue as every other 
part. This equal distribution of the soluble material takes 
place in response to the law of diffusion, that tends to cause 
any soluble substance to become equally distributed through- 
out the liquid in which it is placed. The liquid, in the 
meantime, may remain quite stationary. The process would 
be the same if we were to put in a very small quantity each 
of several soluble substances at the same time. The move- 
ments of one of these substances would not interfere much 
with those of the others. 
Suppose the aniline blue in our glass of water had become 
equally distributed by diffusion. If we could remove some 
of it from the water in one part of the glass, it is clear that 
the particles of dissolved aniline blue would move from the 
other parts toward this point, and if this removal were con- 
tinuous, slow currents of the particles would move in this 
direction from all other parts of the glass. 
We may now understand how the materials from which 
the plant is built up are distributed to the different parts. 
The water absorbed by the root-hairs (101) is not chemically 
pure, but it holds in solution small quantities of various 
soluble matters contained by the soil, some of which are 
used by the plant during growth. As these useful matters 
are removed from the water of the cells, to be built up into 
food, the supply is replenished from the soil, not through 
any power of selection possessed by the plant, but in accord- 
ance with the law of diffusion. In like manner, the food 
