THE WATSONIA 83 



The presumption is that the plant finds it 

 desirable to utilize the short rays of the upper 

 part of the spectrum and the long rays of the 

 lower part — those that stimulate chemical action 

 chiefly, and those that are the greatest convey- 

 ers of heat, respectively — and that the interme- 

 diate rays producing the color green are not 

 needed, hence are reflected or transmitted with- 

 out influencing the plant. 



A possible clew to the reason for this is found 

 in the supposition that the plant needs the short 

 light waves to enable it to carry out its chemical 

 function of transforming water and carbon 

 into sugar, and that this process is facilitated 

 by having the tissues warmed by the long waves 

 of the lower end of the spectrum. 



It has been calculated that the sun beating on 

 a leaf would raise its temperature to a point that 

 would destroy the protoplasm and kill the leaf 

 outright in a very short time were it not for 

 the transpiration of water from the pores of 

 the leaf, through which the temperature is 

 equalized. 



In spite of this danger the sunlight is known 

 to be absolutely essential to the carrying on 

 of life processes, but it is obviously desirable 

 to limit the amount of heat as much as 

 possible. 



