4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



Experiment 2. — The next experiment was very similar to that just 

 described. Here again individual variation was too great to draw any 

 accurate conclusions. 



Experiment j. — In the next experiment three duplicate sets of 

 tomato plants were grown under three different sets of light condi- 

 tions. In front of one lamp a Corning heat-absorbing 212 percent 

 red filter was placed. In front of the other lamp a filter jar containing 

 a M/2 CUSO4 solution was placed. Both filters cut off at 6040 A, the 

 CuSOi solution transmitting light of shorter wave length and the 

 Corning filter transmitting light of longer wave length. Two duplicate 

 sets of cultures were placed between these filtered light sources. A 

 third set was located to the rear of the blue filter light in such a position 

 that the plants received only the full Mazda spectrum. Intensities 

 were measured at the beginning and at the end of the experiment. 

 These average values, together with the plant data for 3 weeks' 

 growth are given in table 2. 



Table 2. — Radiation intensities and azvrage plant data from experiment j 



Average data per plant 



Radiation intensity , ^- ^ 



watts/cm^ Stem Root Total 



Culture I * > ht. length dry wt. 



nos. Red Blue White Total cm cm gram 



I and 2 0055 .0022 .0057 6.3 38 .026 



3 and 4 0028 .0011 .0039 7.4 50 .038 



5 and 6 .0056 .0056 8.0 44 .028 



The greatest amount of dry weight was produced by cultures 3 and 

 4, although the total light intensity was less than under the other two 

 conditions of growth. Here the blue radiation was about 28 percent 

 of the total. Although these data are meager, there is an indication 

 that considerable differences in growth are obtained by manipulating 

 the wave-length distribution as well as the total intensity. 



Experiment 4. — In the next experiment wave-length distribution 

 was further restricted by using neon and mercury grids as light 

 sources. These were constructed in our laboratory by Mr. L. B. 

 Clark, In order to increase the intensities a mirror was placed back 

 of each. Three duplicate cultures were placed between these two 

 light sources, each culture jar containing three tomato seedlings. 

 This increased the number of plants per treatment to six. Because 

 of reflections in the mirrors some red light came from the blue side 

 and some blue light came from the red side of the cultures. As will 

 be seen in table 3, the intensity of radiation was considerably less 

 than in the earlier experiments. The plants were grown for 26 days 



