354 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



energy acting through the green plant to the activities on earth and 

 to the destiny of man. 



Sohir energy received at the surface of the earth is far from being 

 a constant value. It differs greatly from time to time with respect 

 to its duration, intensity, and quality. 



The length of day or duration of sunlight varies with the lati- 

 tude and the season. At the Equator there are approximately 12 

 hours of darkness and 12 hours of light for each day throughout 

 the entire year. The other extreme is reached at the poles, with 24 

 hours of light during the summer season and 24 hours of darkness 

 during the winter season. Intermediate latitudes have sunshine 

 values between these extremes. 



The intensity of sunlight varies inversely as the square of the 

 distance from the sun. Since the earth's orbit is elliptical with the 

 sun at one focus, a difference of approximately 7 percent may be 

 effected with the earth at perihelion in January and at aphelion in 

 July. Other variations in intensity are due to dust particles and 

 water vapor in the atmosphere. An increase of 1 mm pressure in 

 water vapor decreases the radiation intensity about 2 percent. In- 

 tensity also varies with the angular displacement of the sun from 

 the zenith, which is governed by the season and the time of day. 

 Changes in actual amount of energy radiated from the sun also 

 influence the intensity of solar energy reaching the earth's surface. 



The quality or, more accurately expressed, the wave length of 

 sunlight also varies. The white light of the sun with the maximum 

 energy in the yellow becomes richer in red as the sun drops from 

 the zenith to the horizon. This variation in wave-length distribu- 

 tion is due to the lens effect of the earth's blanket of atmosphere 

 and to the differential absorption of light as it passes through it. 

 Sunlight is relatively richer in blue and violet during the summer 

 in the North Temperate Zone than it is in winter. This is like- 

 wise true for high altitudes. 



No wonder a wide range in type of vegetation is encountered over 

 the face of the earth. To be sure, temperature and moisture are 

 important factors in bringing about this variation, but after all 

 variations in temperature and moisture are caused by variations 

 in the solar energy reaching the earth. 



A wealth of information can be obtained by observing the char- 

 acter of plant growth in natural habitats. The accumulation and 

 organization of knowledge concerning the correlation between plants 

 and their environments fall in the realm of plant ecology. A study 

 of the activities of individual plants with emphasis on individual 

 plant processes as these are affected by single environmental condi- 

 tions is restricted to plant physiology. The ecologist endeavors to 



