NATURAL LIGHT FIELDS 



731 



in the depths of the sea ; and the periodicity adaptation shows itself in the 

 distinction between long-day plants of the arctic zone and short-day plants 

 of the temperate and tropical zones. 



The spectral composition and intensities of light fields in different habi- 

 tats of plants have been measured and discussed by several authors, for 

 example, Wiesner (1907), Ursprung (1918), Seybold (1934, 1936) and Egle 

 (1937). The factors that determine the total intensity of the available 

 light (in the photosynthetically important region 400-700 m/x) are the 

 height of the sun over the horizon, the clearness or haziness of the air, 

 cloudiness and the position of the plant in direct sunlight or in the shade. 



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Fig. 22.51. 

 sun ov 



20 30 40 



HEIGHT OF THE SUN, degrees 

 Total irradiation and relative intensity of sunlight in relation to height of 

 or the horizon (after Seybold 1936). Inner scale at left, cal./(cm.- miu.). 



Reflection by the surrounding surfaces may also be of importance, particu- 

 larly for the evergreens (reflection by snow!) and shore plants (reflection 

 by water!) (cf. Egle 1937). Figure 22.50 shows the intensity distribution 

 of the combined light of the sun and the sky for different heights of the 

 Sim over the horizon. The curves show the transition from the red light 

 of the setting sim (ma.ximum intensity at 680 m/x) to the yellow light of the 

 sun in the zenith (maximum intensity at 520 m/i). Figure 22.51 shows the 

 increase in the total energy of the light, from 0.1 cal./cm.^ min., at 10° 

 elevation, to 0.6 cal./cm.^ min., at an elevation of 50°. (The figures are 

 for a surface normal to the direction of the incident light; for a horizontal 

 surface the variations are much wider.) When the sun is obscured by light 

 clouds, the total light intensity decreases to 10 or 20% of its full value (i. e., 

 to 0.1-0.2 cal./cm.- min. at midday). When the sky is entirely overcast. 



