770 BIOLOGICAL EFFECTS OF RADIATION 



blue glass, respectively), his plants were grown in small cupboards under 

 one-sided illumination from the north through a plate glass, the trans- 

 mission of which is not given. Probably all of the plants suffered from 

 want of light. Hence, there is no accurate way of evaluating results 

 under the different screens. If the results can be relied upon at all, 

 apparently water and moor plants respond to the different regions of the 

 spectrum in the same way as do other higher plants. 



Hibben (15) studied the effect of different regions of the spectrum by 

 using an artificial light source. Most of the other investigators have not 

 attempted this because, except on a very small scale, it is not possible 

 to provide an artificial light source which will have a sufficient intensity, 

 after it passes through colored glasses, to satisfy the needs of plants for 

 normal growth. The artificial sources used by Hibben consisted of 

 Mazda lamps and a 480-watt mercury vapor "M" tube. The plants 

 were grown in small compartments. The illumination intensity in 

 the compartments in which Mazda lamps were used is given by Hibben 

 as 350 foot-candles, that in the mercury-vapor-lamp compartment as 

 150 foot-candles. Both these intensities are so far below the intensity 

 of daylight, even in February and March, the time at which his experi- 

 ments were carried out, that it is not surprising that Hibben found 

 that his experimental plants often became taller than those in an ordi- 

 nary greenhouse. The criterion for growth was exclusively height 

 of plants. Furthermore, the tests were continued for only 20 days, 

 which is not long enough for the plants to have become independent 

 of the stored food reserves in the seed. The filters used by Hibben 

 were all Corning glasses. G-38 transmitted wave-lengths down to 

 4500 A; G-124J transmitted down to about 3400 A and was opaque 

 to some of the infra-red ; and G-584 transmitted between 3400 and 6500 A. 

 Of these three, the G-38 glass, which eliminated violet and ultra-violet, 

 gave the tallest plants in the cases of corn, beans, geraniums, nastur- 

 tiums, and marigolds, and the G-584, bluish-green glass, usually gave 

 the shortest. Plants under the mercury-vapor lamps were unhealthy. 



Hibben states that the speed of growth of plants grown under 

 unscreened Mazda lamps and 500 foot-candles intensity was about 

 double that of plants in the greenhouse receiving daylight. While 

 this is probably partly an intensity effect, it is also probably caused in 

 l)art by the deficiency of blue-violet radiation in the Mazda lamps. 

 Hence this greater elongation is similar to what results when plants 

 receive daylight screened through a glass that eliminates blue-violet 

 radiation. It should, of course, be emphasized that an increased rate 

 of elongation usually means a decreased dry weight and a less sturdy 

 plant. 



Many other workers have grown plants entirely under artificial 

 lights. The most extensive experiments have been carried out at the 



