SUN RAYS AND PLANT LIFE— JOHNSTON 363 



air, the two in the background were enclosed by glass cases 5 feet 

 high. Two pieces of fly netting were stretched across the top of 

 each to reduce wind action. A pipe carrying a mixture of air and 

 carbon dioxide opened into the plot on the right. At the end of the 

 experiment the growth in this carbon-dioxide-treated plot was com- 

 pared to that in the enclosed control plot on the left and to that in 

 the open control plot in the foreground. 



The appearance of the three crops at harvest is shown in plate 3, 

 figure 2. A received air enriched with carbon dioxide to about four 

 times that of normal air. B was grown in the enclosed control plot, 

 and O in the open. The experiments of the second summer were very 

 similar. It was shown in these experiments that air enriched with 

 carbon dioxide (1) increased the tillering of the wheat, (2) greatly 

 increased the weight of straw, increased (3) the number and (4) 

 weight of heads, (5) increased the number of grains produced, and 

 (6) slightly delayed the time of heading. 



The practical application of aerial fertilization with carbon diox- 

 ide and the source of supply of carbon dioxide in sufficient amounts 

 for field work are still unsolved problems. Its application to green- 

 house culture appears to be more promising. 



WAVE-LENGTH EFFECTS 



The chemical reaction to which attention has been draAvn is, per- 

 haps, the most important in the whole world, for life itself would 

 perish without photosynthesis. It is therefore interesting to examine 

 it from all points of view. On the chemical side, much remains to 

 be done. The complexity of organic chemical reactions, the little- 

 understood effectiveness of so-called catalysts, the behavior of en- 

 zymes, of colloids, of hormones, altogether make up a field of research 

 of the utmost interest, but of extreme difficulty, for the student of 

 plant growth. 



Furthermore, photosynthesis takes place under the influence of 

 light. Its energy is derived from radiation. The question immedi- 

 ately suggests itself, "What rays are utilized in this reaction?" Al- 

 though many qualitative studies of this problem have been made, 

 there is but little quantitative data on the subject, especially with 

 economic plants. Mr. Hoover (1937), of the Smithsonian Institu- 

 tion, has made quantitative determinations of the dependence of an 

 important higher plant — wheat, in this case — on the wave length 

 of light for the assimilation of atmospheric carbon dioxide. He used 

 the ingenious Christiansen filter (McAlister, 1935) to separate nar- 

 row bands of the spectrum from the beams of a group of Mazda 

 lamps surrounding the tall glass tube within which the wheat was 

 grown. Atmospheric temperature and moisture Avere controlled. A 



