318 GROWTH OF PLANTS 



fruits, and general storage organs, (i) Decrease in fresh weight and dry 

 weight and an increase in percentage of moisture. ( j) Considerable decrease 

 in starch and total carbohydrates, and generally an increase in total nitro- 

 gen; often an increase in soluble nitrogen compounds. 



The degree to which these different effects were produced varied \\dth 

 different species, but all species, aside from the abundance of chlorophyll, 

 had an etiolated appearance. 



When all wave lengths shorter than 472 m/x were removed, the same 

 effects were produced as listed above, but to a somewhat lesser degree. 



When only ultraviolet rays were eliminated, none of the foregoing results 

 were obtained with any of the plants used, although there was a small in- 

 crease in length of stems in all species except buckwheat, as compared with 

 plants receiving these rays. Tomatoes, petunias, Sudan grass, and sun- 

 flowers bloomed somewhat earlier than they did under any other condi- 

 tions. In general, there was very little difference between plants that 

 received all the rays of the spectrum of daylight and those from which only 

 ultraviolet rays were eliminated. 



The results obtained with plants from which all wave lengths shorter 

 than 529 or 472 m^t were eliminated are somewhat similar to those obtained 

 when plants are grown under greatly reduced light intensity. That light 

 intensity was not an important factor in the present experiment is proved 

 by the fact that normal, vigorous growth was obtained when the plants 

 received the full spectrum of daylight at an intensity which was at all times 

 lower than that of the house in which all wave lengths shorter than 472 m^ 

 were removed, and only slightly greater than that of the house in which 

 wave lengths shorter than 529 m^u were eliminated. 



The results as a whole indicate that, while ultraviolet rays are not indis- 

 pensable, the blue-violet end of the spectrum is necessary for normal, 

 vigorous growth of plants. 



In a later study ^ the glass on spectral house 3, the ultraviolet-excluding 

 glass, was replaced by a glass that cut out the red rays and transmitted 

 rays between 585 and 348 niju. This glass gives low transmission of solar 

 energy, about 1 1 per cent in contrast to 37 per cent for house 5, the weakest 

 transmission of the original spectral houses. The transmission of this glass 

 is also mainly in the blue-violet. Fig. 131 shows (top) petunias grown in 

 greenhouses 2, 3, and 5 and (bottom) four o'clocks grown in greenhouses 

 2, 3, and 5 and outside in shade and in full sunlight. The four o'clock and 

 petunia plants gro^\^l in house 5 are spindling, as was the case with four 

 o'clock and soybean in Fig. 129. Both the plants grown in house 3 with 

 blue-violet light are short and sturdy but show little growth. It is possible 

 that sufficient intensity of light in the blue and violet will give normal 

 plants and good growth. 



The window glass in spectral house 1 was later replaced by Aklo with 

 very low transmission in the infrared. The infrared transmission of this 

 glass is described by Arthur,-- ^■'^ but no results on its use for growing plants 



