SUN RAYS AND PLANT LIFE— JOHNSTON 



357 



There is fairly good agreement between these values and those of 

 actual sunshine duration. The temperature values showed a rather 

 decided seasonal variation, with high summer values and low winter 

 ones. 



The climatic efficiency values within the cheesecloth chamber ap- 

 pear, in general, to have been lower than those in the unshaded 

 greenhouse, the exceptions being the values for stem height and for 

 leaf area for the periods about June 21. 



Although the interpretation of the plant values in terms of those 

 derived from the instruments offers many difficulties, nevertheless 

 several striking features of this environmental complex are regis- 

 tered in the records of both plants and instruments. The one to 



ELECTROMAGNETIC RADIATION 



COSMIC RAYS ? 



CENTIMETERS 



4220 3900 ANGSTROMS 



YELLOW GREEN 



Figure 2. — Diagram showing position of infrared, visible, and ultraviolet radiation in the 

 great electromagnetic spectrum. Data from Deming and Cottvell and from "Handbook 

 of Chemistry and Physics." 



wliich attention is especially directed is the general agreement to be 

 observed between the radiation values and those of diy weight and 

 leaf area. 



These general relationships between sunlight and plant growth are 

 interesting enough to warrant a more detailed examination by re- 

 viewing briefly the effects of the various components of sunlight 

 upon the different physiological processes that take place in plants. 

 The sunlight which we shall consider is actually a very small fraction 

 of the great electromagnetic spectrum. 



As will be seen in figure 2, this immense series of wave lengths 

 extends from far beyond the short gamma waves produced from 

 radioactive matter such as radium to the long wireless waves. The 

 magnified portion shown below includes the wave lengths of the 

 visible spectrum from red to violet. This, together with a section 

 in the infrared and another in the ultraviolet, comprises the wave- 

 length regions for our present discussion. 



