„ │23│ 15│ 37│ :11│ November │ 2│ 16│ 20│ :10│ „ │12│ 15│ 45│ :11│ „ │17│ 14│ 54│ :12│ „ │22│ 13│ 44│ :13│ „ │27│ 12│ 14│ :15│ December │ 2│ 10│ 25│ :16│ „ │ 7│ 8│ 21│ :18│ „ │12│ 6│ 5│ :21│ „ │17│ 3│ 41│ :23│ „ │22│ 1│ 12│ :26│ │ │ _Sun slow_: +│ │ „ │27│ 1│ 17│ :28│ ─────────┴──┴───────┴───────┴──────┴────── =87. Place.= The effect of latitude upon the sun’s altitude, and the consequent light intensity have been discussed in the pages which precede. Latitude 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 intensity 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 almost 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[5] has found, moreover, that photosynthetic activity in _Elodea_ is not increased beyond the normal in sunlight sixty times concentrated. In consequence, it is entirely unnecessary to take account of different altitudes in obtaining light values.