76 PLANT PHYSIOLOGY AND ECOLOGY 



when the angle that it makes with the surface of the earth is 

 greatest, and the lowest when this angle is the least. At equal 

 distances from either maximum, e.g., at 8 a.m. and 4 p.m., or 

 on March 21 and September 23, the angle is the same. The effect 

 of angle upon light intensity is due to the absorption of the light 

 rays by the earth's atmosphere. This absorption is greatest 

 near the horizon, where their pathway is the longest, and it is 

 least at the zenith. In other words, the absorption is greatest 

 at sunrise and sunset, least at noonday; greatest in December, 

 least in June. Considering the two at the same time, maximum 

 sunlight occurs at noon on June 22, minimum sunlight at sun- 

 rise or sunset on December 22. 



From the foregoing it is clear that the variation in light due 

 to time must be taken into account. In critical investigation, 

 it is desirable to compute the error arising in this way, and to elim- 

 inate it from the reading. For ordinary purposes, it suffices to 

 make readings directly comparable by restricting the times at 

 which they are made. In a growing period of six months begin- 

 ning March 21 and closing September 23, the noon intensities 

 are respectively .98 and .93.^ Between 9 a.m. and 3 p.m. during 

 this period, the range would be from .82 to .98, a difference that 

 is practically negligible. Hence, in all ordinary study, the value 

 of readings taken in full sunlight between 9 a.m. and 3 p.m. from 

 March 21 to September 23 may be regarded as unity. Conse- 

 quently, readings made in shady habitats during the same period 

 may be compared with them directly without making allow- 

 ance for the slight error. Naturally, when readings are taken 

 simultaneously, no error exists, and the same is practically true 

 of readings made between 10 a.m. and 2 p.m. 



95. The effect of altitude. Altitude affects the amount of 

 light by decreasing the distance which the rays must travel through 

 the atmosphere, and thus decreasing the absorption. This influ- 

 ence is much smaller than has been commonly supposed. It is 

 estimated that 20% of a light ray is absorbed before it reaches 

 sea-level. At the top of Pike's Peak (4267 meters high), the 

 absorption is 11%. In the one case the light is 80% of that 

 which enters the atmosphere, in the other 89%. Consequently 

 the maximum intensity at sea-level and at the summit of Pike's 

 Peak is .98 and 1.09 respectively. The difference between them 



^ Research Methods, 57. 



