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- 

 Prgyj (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 

 CO2 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 CO2 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 CO2 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. 



CO2 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 CO2 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 CO2 in 

 the air, within limits, the more can enter them and be used for food 

 making. 



CO2 in water. — In the water of quiet pools and lakes, as well as in 

 slow streams, the amount of CO2 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 CO2 is very soluble 

 in water (up to volume for volume at ordinary temperatures), it may 



