26 



THE FLOWERING PROCESS 



400 600 800 1000 1200 1400 1600 » 1800 2000 



U.V. I ^ I c I Red I Infra-red. 



m g WAVE LENGTH IN MILLIMICRONS 



Figure 3-1 

 The solar spectrum. Curve 1 shows the normal solar energy distribution 

 of radiation estimated for outside the atmosphere. Curve 2 shows the 

 same at the surface of the earth on an average day in Washington, 

 D.C., with 1.37 cm of precipitable water in the atmosphere and the sun 

 25 degrees from the zenith. Curve 3 shows the shift towards the red 

 when the sun is 78.7 degrees from the zenith (altitude of 11.3 degrees), 

 producing an air mass about 5 times that of Curve 2. Other conditions 

 about the same as for Curve 2. Redrawn from Otis F. Curtis and 

 Daniel G. Clark, 1950, An Introduction to Plant Physiology, McGraw- 

 Hill, New York. 



The energy of this infra-red light warms our planet, although so far 

 as we know, organisms do not respond to it photochemically. It is 

 probably a Uttle more efficient in warming the earth than the visible 

 light, because infra-red is absorbed more efficiently by most things 

 on the surface of the earth. Water, for example, absorbs much of the 

 infra-red, while the visible is mostly either transmitted or reflected. 

 Carbon dioxide also absorbs infra-red Hght but not visible hght. 



Note that the peak of the midday curve falls approximately in the 

 region of green light. Our eyes are most sensitive to green light, but 

 most plant processes are relatively insensitive to light of these wave- 

 lengths. Quality changes that occur in nature are usually considered 



