8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



the infra-red was removed. A comparison of the dry weights shows 

 the greatest growth to have occurred in chamber 4 and the least in 

 chamber 2. In a subsecjuent experiment, similar to this one except 

 that the temperatures were higher, the same order of growth, as 

 measured by dry weight, existed between the four chambers. That 

 is, in order of plant dry weight the series in this and a subsequent 

 experiment was 4, 3, i, 2 (high visible plus infra-red, high visible 

 only, low visible plus infra-red. low visible only). It is also rather 

 interesting to note that the growth of plants in chamber 2 (low 

 visible) was very similar to that in the north window of the laboratory. 

 Also there was a good deal of similarity between the plants in cham- 

 ber 3 (high visible) and those grown in the west window of the tower. 



A greater amount of water was absorbed by the plants exposed to 

 the greater light intensities — those in chambers 3 and 4. It should 

 also be noted that in chambers i and 4, where the proportion of red 

 and infra-red was greater, the plants elongated much faster than 

 those in chambers 2 and 3. Although there was little difference found 

 in the average number of leaves of plants in the four chambers, 

 there is an indication that more leaves were produced in the more 

 intense light. 



Attention is directed to the order of greenness of the plants grown 

 under these six conditions of illumination. Those grown in chambers 

 2 and 3, which received only visible radiation, and in the north window 

 were greener than the others. The lower leaves of the plants in 

 chamber 4, where the radiation was more intense and included infra- 

 red, had turned yellow. Those in chamber i, while not as yellow as 

 those in 4, were far from a healthy green color. 



In the lower part of the table several derived values are tabulated. 

 Water requirement is here considered as the amount of water ab- 

 sorbed by the plant during the two wrecks of growth per unit of 

 total dry matter. Plants in chambers i and 4, where infra-red radia- 

 tion was present, were more economical in their use of water than 

 the others. The ratios of root to top dry weights indicate that some- 

 what heavier roots in proportion to tops occur without infra-red. 



The internodal index was determined by dividing the height of the 

 plant by the number of its leaves. It gives a relative index of the 

 length of internodes. This index as well as the ratio of final to 

 original height of the plants shows that much less elongation occurs 

 without infra-red. 



Plate 3 shows the general appearance of the six groups of plants 

 after two weeks of growth under the conditions of light mentioned 



