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 cf 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 Elodea is not increased beyond the normal in 
sunlight sixty times concentrated. In consequence, it is entirely unneces- 
sarv 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 go°, 
a square meter of surface will receive the maximum intensity, 1. 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 and 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 
1REINKE, J. Bot. Zeit., 41:713. 1883. 
