768 BIOLOGICAL EFFECTS OF RADIATION 



height in this house, while sunflowers, buckwheat, petunias, and Sudan 

 grass had the lowest final height. The soybeans actually became 

 twiners. All stems, however, were much thinner in this house and had 

 fewer branches. The leaves were curled or rolled, and both stem and 

 leaf tissues were poorly differentiated, having thin-walled cells that 

 were less compact than the cells of leaves and stems of plants receiving 

 the full spectrum. There was a poor development of vascular tissue and 

 of all storage organs. Time of flowering was delayed and the number of 

 flowers, fruits, and seeds greatly reduced in house 5. Fresh and dry 

 weights decreased and percentage of moisture increased. Chlorophyll 

 developed normally, but anthocyanin development was greatly reduced. 

 There were also important changes in carbohydrates and in nitrogen 

 compounds. 



The same effects were produced to a lesser degree in house 4 which 

 eliminated all wave-lengths below 4720 A, but none of them were 

 produced when only ultra-violet radiation was eliminated (house 3). 



The effects produced by eliminating the blue-violet end of the spec- 

 trum were similar to etiolation produced by greatly reduced intensity. 

 In these experiments, however, there is unquestionable proof that this 

 etiolation was produced by quality of light rather than by total intensity. 

 The results as a whole indicated clearly that the blue-violet end of the 

 spectrum is necessary for normal, vigorous growth of plants. They also 

 indicated that ultra-violet radiation is not necessary, although it may not 

 be without influence. Any influence it may have, however, is rather one 

 of checking elongation of stems, than one of promoting growth in length. 



Popp's work was the first in which intensity and temperature were 

 sufficiently controlled to enable one to be certain that the effects produced 

 were actually effects of quality of light. It is none the less interesting 

 that his work does to some extent agree with the earlier findings, namely, 

 that the red end of the spectrum promotes stem elongation, while the 

 blue-violet end checks it. However, the stems produced in the presence 

 of blue-violet rays, while they may be shorter, are much heavier and actu- 

 ally represent more growth in weight. This latter fact was generally 

 overlooked by the earlier workers. 



Popp, in his work, considered the possibility of growing some plants 

 under blue-violet light in the absence of the red end of the spectrum. 

 This was not done, however, because it was impossible to obtain a screen 

 which would eliminate the red end of the spectrum and at the same time 

 transmit sufficient total energy to satisfy the needs of the plant. Later, 

 Shirley (43), working in the same houses Popp used, did use in one of the 

 houses a blue glass (Corning G403 ED) which transmitted wave-lengths 

 between 3740 and 5850 A. This glass, however, transmitted only 

 about 10 per cent of the total energy of sunlight; hence he had to cut down 

 the intensity of all houses to this low figure to get comparable results. 



