CLIMATIC FACTORS 



105 



Laneous observations in two adjacent habitats such as within and with- 

 out a forest (Fig. 57). By observing the time required for the paper to 

 reach the normal shade under the trees and in full sunlight the results 

 may be expressed as a fraction or as a percentage. 



The decrease of light under different layers of a forest of Betula in 

 North Germany is given by Hueck: in the open, 100 per cent; under the 

 tree canopy at 150 cm. above the soil, 34 per cent; under the Pteris 

 aquilina layer at 39 cm., 4 per cent; under the Vaccinium lyiyrtillus 

 layer at 5 cm., 0.7 per cent; and under the Vaccinium only a few small 

 mosses were growing {cf. Zon and Graves, 1911). 



In an effort to measure with relative accuracy the different parts of 

 the visible spectrum, Klugh (1925) has devised an instrument in which 



A.M.b 



6 P.M. 



Fig. 57. 



-Simultaneous light curves made on May 23 in the open (a) ; in the forest under 

 Alnus (6); under Quercus (c) ; under Fa^ws {d). (After Vallin.) 



panchromatic photographic plates are exposed beneath a set of neutral 

 percentage filters. After development, the results from the exposures 

 at lower intensities are read against those from exposure at the highest 

 intensity, and the readings expressed directly in percentages. Klugh 

 (1927) has also presented an excellent critical review of methods in 

 ecological photometry. 



Light Climates. — It is well known that shade plants of the south, or 

 of the lowlands, may grow wholly in the open farther north or on moun- 

 tains. Their light requirements change with latitude and altitude. 

 Light measurements such as Wiesner (1907) made for Betula alba, B. 

 nana, Acer platanoides, and other species in Central or northern p]urope 

 and in the arctic show that the minimum light requirement increases 

 rapidly from south to north. It was l-i^ for A. platanoides in Vienna, 

 3>'37 in Hamar (Norway), 3-^8 in Drontheim, 3^5 in Tromso. From this 



