CHEMISTRY AND PHYSICS OF PLANTS. 19 
151. How the Food is Transported in the Plant.—Once 
within the plant-body, the food-materials diffuse to all 
watery parts, in the case of the larger terrestrial plants 
rising through the stem to the leaves. By diffusion there 
is a constant tendency toward an equal distribution through- 
out the plant of the solutions which enter it; and if there 
were no disturbing chemical reactions taking place, such a 
condition would in most plants be soon reached. It is quite 
probable, indeed, that this actually happens for certain 
substances which are found in solution in the soil or water, 
and which, entering plants, diffuse through them to all 
parts, but not being used they soon reach a state of equal 
diffusion, which is only slightly disturbed by the extension 
of the plant-body by growth. The diffusion of food-mate- 
rials throughout terrestrial plants is aided by the evapora- 
tion of water from the leaves, thus causing a strong upward 
movement of the water which contains the various solutions 
of food-matter; but it is not dependent upon evaporation, 
for diffusion takes place under conditions which preclude 
evaporation. 
152. Starch-making, or Assimilation —Most of the food- 
materials of plants can be directly used by the protoplasm. 
Thus the oxygen and water, and the nitrates, sulphates, 
etc., mentioned above may be at once made use of by the 
protoplasm for its own nourishment. It is not so, however, 
with the carbon dioxide. It cannot be used directly as 
food, but must first undergo a special preparation. It must 
“be broken up and recombined along with the elements de- 
rived from water so as to form a new compound which the 
protoplasm can digest. This new compound is starch, or 
something much like it, and so we may call this prepara- 
