332 



GROWTH OF PLANTS 



ment in the gantry crane houses and in continuous Hght, while the maximum 

 height of the top occurred in the 17-hour day. Increased development of 

 the underground storage organs on long days is in contrast to the behavior 

 in the Jerusalem artichoke and dahlia, which develop such storage organs 

 on short days, but it agrees with Arthur's findings, mentioned above, that 

 potato tuberizes well under long days if other conditions are favorable. 



Table 35. Spectral Transmission of the Houses 



An anatomical study ^^ was made of plants grown in several spectral 

 greenhouses and outdoors as a check. The spectral transmission of several 

 of these houses has already been described under the topic "Spectral Green- 

 houses," but since the glass on one of these houses was changed, the rays 

 transmitted by each house are given in Table 35 and the light and radiant 

 energy transmitted by each house in Table 36. Table 37 shows the relative 



Table 36. Intensities in the Various Situations 



effectiveness of the several lights on the development of stems, leaves, and 

 roots of Mirahilis jalapa, Brassica rapa, Helianthus cucumerifolius, and 

 Glycine soja. Pfeiffer points out that both the intensity of illumination and 

 the quality of the light varied. The hght transmission (foot-candles) m the 

 total spectrum, visible spectrum, and minus-blue houses was nearly identi- 

 cal, and exceeded 50 per cent of that outdoors. The illumination was very 

 low m the blue house, about 8 per cent of that outside, and about tmce 

 that of the shaded house. The red house was about 31 per cent of that 

 outside. This variation in intensity makes the results hard to interpret. 

 This was partly overcome in the later work by Shirley, already discussed, 

 where cloth screens were used to equalize more nearly the intensity in the 

 several houses. Pfeiffer points out that plants grown in the full spectrum 



