68 Diseases of Greenhouse Crops 



cities. But more extensive investigations are needed 

 to convince the grower of the practicability of the 

 attempt. 



Effect of Shading 



It has been shown in the previous chapter that 

 certain light rays, such as green or blue, are detri- 

 mental to plant growth. On the other hand, the 

 normal sun rays from outdoors or as they come 

 through white glass are most conducive to normal 

 plant culture. The practical man, however, real- 

 izes that at certain times of the year, especially dur- 

 ing the summer months, the white glass must be 

 shaded to prevent an excess of sunlight. This is 

 accomplished by whitewashing the glass. That this 

 procedure is necessary no one can question. How- 

 ever, it must be admitted that the method itself is 

 still a crude one, inasmuch as the various plants 

 in the hothouse are subjected alike to the same 

 amount of shading. Shantz* has shown that while 

 a certain amount of shading is beneficial to plant 

 growth, yet not all plants are benefited alike by 

 this treatment. 



Effect of Different Light Intensities on 

 Plants 



The work of Shantz distinctly shows that all 

 plants do not tolerate the same amount of shading. 

 To prove this he grew various crops in a bed cov- 



• Shantz, H. LeRoy, U. S. Dept Agr. Bur. PI. Ind. Bui. 379: 7-29, 

 1913. 



