CONTROLLED ENVIRONMENTAL CONDITIONS 295 



at a cost of 15 cents a square foot day. As we shall see later, sodium vapor 

 lights supplemented -with, mercury vapor or Mazda lights give a good grow- 

 ing light for plants at 3 cents per square foot day. This study indicated 

 the efficiency of the sodium vapor lamp and led to the experiments reported 

 in the next section. 



The authors make the following statement concerning these four lights 

 and the absorption of their rays by chlorophyll: ^^- p-^'" "A consideration of 

 the absorption spectrum of chlorophyll shows no relation between the 

 emission bands of the various lamps, the absorption bands of the chlorophyll 

 pigments, and the efficiency of the lamp in producing dry weight of plant 

 tissue. The sodium lamp was found most efficient, w4th the main output 

 of energy at wave lengths 588 and 589 m/x, a point at which chlorophyll 

 absorption is near the minimum. The neon lamp was second in efficiency 

 with the main output band near the maximum of chlorophyll absorption in 

 the red-oi'ange region. The mercury vapor lamp was least efficient, yet has 

 much of the energy output in the blue-violet region where chlorophyll 

 absorption is maximum. The sodium lamp has an efficiency of 45 lumens 

 per watt and a remarkably low power loss in the auxiliary transformer unit 

 of only 25 watts, as compared with a current consumption of 200 watts in 

 the arc itself." This is understandable when it is realized, as is well kno^^'n, 

 that photos5Tithesis makes very inefficient use of light at the best. It should 

 be emphasized that the plants in this work were gro^\^l under the lights 

 for only 8 to 11 days. Plants grown continuously for a long period under 

 sodium vapor lamps unaccompanied by any other illumination are injured. 

 The three gaseous discharge lamps produced greener leaves and a lower 

 ratio of stems to leaves than the Mazda lamp. 



Supplemental sodium vapor lamps as constant light source. Since 

 sodium vapor lamps had proved a good and cheap light source for growing 

 seedlings over a short period, they were tried ^^ for growing plants over a 

 long period under continuous illumination. Plants do well for a short time 

 under this light but after two months of continuous exposure only a few 

 yellow leaves remained at the tip of the plants. This is especially true at 

 temperatures somewhat above 70° F (21° C) and far less marked at 63° F 

 (17° C). These plants recover when still under continuous illumination 

 from the sodium vapor lamp if they are illuminated two hours daily ^dth 

 85-watt capillary mercury vapor lamps with the temperature at 63° F 

 (17° C). The final set-up for these experiments is sho^NTi in Fig. 117.'^ Four 

 10,000-lumen sodium vapor lamps are mounted in the form of a square two 

 feet on a side and a small 85-watt capillary mercury arc lamp, type H-3, 

 is mounted in the center of the square, all with reflectors as sho^vTi. The 

 sodium vapor lights burn continuously and the mercury vapor lamp is turned 

 on two hours each day. Several kinds of plants (begonia, gardenia, cotton, 

 geranium, and buckwheat) did well under this illumination. It did not 

 prove satisfactory for the tomato, which does not do well under any con- 

 stant illumination yet tested. Fig. 118 shows several kinds of plants gro\\Ti 



