LIGHT 59 



87. Place. The effect of latitude upon the sun's altitude, and the conse- 

 quent light intensity have been discussed in the pages which precede. Lati- 

 tude has also a profound influence upon the duration of daylight, but the 

 importance of the latter apart from intensity is not altogether clear. The 

 variation of intensity due to altitude has been greatly overestimated; it is 

 practically certain, for example, that the dwarf habit of alpine plants is not 

 to be ascribed to intense illumination, since the latter increases but slightly 

 with the altitude. It has been demonstrated astronomically that about 20 

 per cent of a vertical ray of sunlight is absorbed by the atmosphere 

 by the time it reaches sea level. At the summit of Pike's Peak, which is 

 14,000 feet (4,267 meters) high, the barometric pressure is 17 inches, and 

 the absorption is approximately 11 per cent. In other words, the light at 

 sea level is 80 per cent of that which enters the earth's atmosphere; on the 

 summit of Pike's Peak it is 89 per cent. As the effect of the sun's altitude 

 is the same in both places, the table of curves on page 57 will apply to 

 both. Taking into account the difference in absorption, the maximum in- 

 tensity at sea level and at 14,000 feet on the fortieth parallel is .98 and 1.09 

 respectively. The minimum intensities between 8:00 a.m. and 4:00 p.m. of 

 the growing period are .64 and .71 respectively. The correctness of these 

 figures has been demonstrated by photometer readings, which have given al- 

 most exactly the same results. .Such slight variations are quite insufficient 

 to produce an appreciable adjustment, particularly in structure. They are 

 far within the efficient difference, and Reinke^ has found, moreover, that 

 photosynthetic activity in Elodca is not increased beyond the normal in 

 sunlight sixty times concentrated. In consequence, it is entirely unneces- 

 sary to take account of different altitudes in obtaining light values. 



The slope of a habitat exerts a considerable effect upon the intensity of 

 the incident light. If the angle between the slope and the sun's ray be 90, 

 a square meter of surface will receive the maximum intensity, i. At an 

 angle of 10, the same area receives but .17 of the light. This relation be- 

 tween angle and intensity is shown in the table which follows. The influence 

 of the light, however, is felt by the leaf, not by the slope. Since there is 

 no connection between the position of the leaf ?.nd the slope of the habitat, 

 the latter may be ignored. In consequence, it is unnecessary to make al- 

 lowances for the direction of a slope, viz., whether north, east, south, or 

 west, in so far as light values are concerned. The angle which a leaf makes 

 with its stem determines the angle of incidence, and hence the amount of 

 light received by the leaf surface. This is relatively unimportant for two 

 reasons. This angle changes hourly and daily with the altitude of the 



iReinke, J. Bot. Zeit., 41:713. 1883. 



