THE ROLE OF LIGHT 347 



ing indirect increase In the carbon dioxide content of the lower layers of the 

 atmosphere. Application of fertilizers not only favors increased growth of 

 the higher plants, but also leads to an increased development of soil micro- 

 organisms. The increased respiration of these organisms results in measurable 

 increases in the carbon dioxide concentration of the stratum of the air close 

 to the ground level. This local enrichment in the carbon dioxide content 

 of the atmosphere will favor photosynthesis in all low-growing species of 

 plants, and often results in lower leaves or branches on a plant being exposed 

 to considerably higher concentrations of carbon dioxide than other leaves or 

 branches only a few feet higher above the soil level. On well fertilized fields 

 the production of carbon dioxide by soil respiration during a tvventy-four 

 hour period may equal or exceed the consumption in photosynthesis during 

 the daylight hours. Over unfertilized fields, pasture lands, etc. the air close 

 to the ground is often relatively low in carbon dioxide concentration, especially 

 during the daylight hours when environmental conditions are favorable to 

 the occurrence of photosynthesis at appreciable rates. 



Similarly in forests the carbon dioxide concentration of atmospheric layers 

 close to the soil surface is often several times as great as in layers higher up, 

 due to intensive soil respiration. This favors photosj^nthesis and growth in 

 herbs, shrubs, and j^oung trees which do not rise far above the floor of the 

 forest. On the other hand, in the atmosphere on a level with the crowns 

 of the trees the carbon dioxide content is sometimes considerably less than 

 the average atmospheric value during the hours when photosynthesis is in 

 progress. 



The Role of Light. — The energy stored by green plants in the molecules 

 of simple sugars during photosynthesis can be supplied only by light, and 

 in the absence of illumination photosynthesis fails to occur. Any source of 

 radiant energy which furnishes wave lengths within the range of the visible 

 spectrum will induce photosynthesis, provided its intensity is sufficiently great. 

 Although a few of the longer wave lengths of ultraviolet apparently are 

 effective in photosynthesis, in general this process can occur only in radiation 

 of the visible part of the spectrum. Under natural conditions sunlight, either 

 direct or reflected from the sky, other objects, etc. is the only source of radiant 

 energy supplying wave lengths which can be used in photosynthesis. Photo- 

 synthesis will occur under electric lights or other artificial sources of illumina- 

 tion if of sufficient intensity. Such light sources are often used in experimental 

 work on photosynthesis and to some extent in greenhouses as supplementary 

 sources of illumination. 



Light, like all forms of radiant energ>^ varies in intensity, quality, and 

 duration (Chap. XIX), and the influence of this factor upon photosynthesis 



