364 PHYSIOLOGY 
tinctive term photosynthesis for the process that is peculiar to green 
plants, leaving the term assimilation to be applied to the same process 
in both plants and animals; namely, to the transformation of foods of all 
kinds into the actual living stuff. 
As photosynthesis requires a supply of certain substances, which re- 
appear in more elaborate form, and acts through certain structures, which 
require a supply of energy for doing the work, the making of carbohy- 
drates may be described appropriately in terms of a manufacturing pro- 
cess. There are (i) the raw materials, (2) the laboratories, (3) the en- 
ergy, (4) the products and the process. 
(i) THE RAW MATERIALS 
Carbon dioxid. The raw materials needed have already been 
named, carbon dioxid and water. Carbon dioxid exists everywhere in 
the air, in the ratio of about 3 parts in 10,000, and its nearly uniform 
distribution is assured by the convection currents (winds) that stir the 
atmosphere. Only in the neighborhood of cities or other places where 
CO a is being produced in quantity is there temporarily an excess. By 
decomposition of rocks, burning of fuel, decay of organic matter, and 
respiration of plants and animals, the supply of CO 2 is maintained, though 
great quantities are removed from the air by green plants. The amount 
is constant, so far as can be known historically, though there is geological 
evidence that in earlier periods of the earth's development CO 2 existed 
in much larger and also in smaller quantities than now, since enormous 
amounts have been fixed in beds of limestone, and later released by 
weathering. 
CO 2 near the ground. On quiet days there is a layer of air near 
the ground where the proportion may rise much higher (10 to 12 
times as much), owing to the diffusion of CO 2 from the soil, where 
it is being evolved by the decomposition of organic matter through 
the agency of bacteria, etc. Perhaps turf-forming and rosette plants 
profit from the lowly position of their leaves, since the more CO 2 in 
the air, within limits, the more can enter them and be used for food 
making. 
CO 3 in water. In the water of quiet pools and lakes, as well as in 
slow streams, the amount of CO 2 dissolved is much greater than in the 
air. It is produced by the host of organisms living in the waters and 
by decay, and is also dissolved from the air. As CO 2 is very soluble 
in water (up to volume for volume at ordinary temperatures), it may 
