LIGHT AND PLANT GROWTH BRACKETT AND JOHNSTON 261 



yellow, yellow to green, and green to blue, it is desirable to expose 

 the plant to a narrower wave-length band. If we look at white light 

 as it is spread out into the spectrum by a prism, we see that the red 

 changes gradually into yellow, the yellow into green, and green 

 into blue, and that an indefinite number of difl'eront ])ure colors 

 actually exists. By using an instrument called a monoclirometer, it 

 is possible to select a more definite shade of yellow or green, and 

 instead of using just three wide color bands, we could divide the 

 spectrum into oO or lOO purer colors or bands, of more restricted 

 wave-length range. If we plot, then, the intensity of each shade 

 required to balance the phototro})ic influence of a given standard 

 lamp against the wave length, we can obtain a group of 50 or 100 

 points instead of the three obtained in the preliminary experiment. 

 The equipment for this more elaborate undertaking is under 

 construction. 



In the general study of the very complicated phenomena of plant 

 growth, light enters as an important factor in many ways. Supply- 

 ing, as it does, the necessary energy for the process of converting 

 carbon dioxide into sugars and starches, its relation to this process, 

 termed photosynthesis, is one of greatest interest. As the energ}' 

 which we derive from our food was originally stored up by this 

 process of photosynthesis, an understanding of the process is of 

 the greatest importance to our existence. Experiments are being 

 imdertaken, designed to study the ell'ect of different colors of light 

 upon plant growth, together with observations as to the amount 

 of carbon dioxide utilized under rigidly controlled conditions. In 

 these experiments nutrient solutions instead of soil are used because 

 of their reproducibility. Lights similar to Neon signs, but yielding 

 more nearly a single wave length, replace the usual sunlight. It is 

 hoped that these experiments will throv,- some light upon the little 

 understood process of photosynthesis. 



Among the difficulties which face the experimenter in studying 

 these phenomena is the fact that he does not know the construction of 

 the very complicated molecules which are present in living tissue. 

 Chemical analysis yields information as to the elements which make 

 up tliese complicated molecules, but the usual methods of observation 

 do not offer enough data for determining the position of the various 

 atoms, or the forces which hold them together. Study of the light 

 which such molecules emit or absorb offers a wealth of new data 

 which may serve as a key to a more complete understanding of those 

 molecules which are of pivotal importance. In order to understand 

 how such information can be gained, it may be well to make some 

 simple comparisons. When we hear a sound we are immediately 

 able to say that the source of the sound is a bird, a whistle, or a horn, 

 as the case may be. We do so by associating the characteristic 

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