CLIMATIC FACTORS 86 



The climatological measurement of nocturnal radiation is made 

 by means of a minimum thermometer placed on bare ground and almost 

 covered with earth. Or it may be placed horizontally just above a 

 covering of short grass. The difference between the minimum temper- 

 ature on the ground and 1.5 m. above gives a measure of the intensity 

 of radiation. Such measurements have only rarely been made. They 

 would throw light on many peculiarities of vegetation in regions with 

 cold winters and little snow. The minima of the surface of the ground 

 furnish an important selective factor for the sensitive seedlings of 

 trees and shrubs. 



Temperature Graphs. — In order to determine the amount of heat 

 which plant communities actually receive, it is necessary to have self- 

 registering apparatus in the habitat.^ The problem is complicated by 

 the fact that, strictly speaking, each layer of vegetation has its own 

 peculiar temperature requirement. The upper layers influence the 

 lower, making the climate more equable. 



G. Kraus (1911) carried out minute (though not self-registering) 

 determinations of the air temperature of the habitat in the Wellenkalk 

 region of Wiirzburg. From his data we have confirmation of the well- 

 known fact that air temperature even in a very small area is subject to 

 great local variation and often to very sudden changes. Biihler (1918) 

 has given us temperature graphs throughout the year for several layers 

 of vegetation in an economically important forest community. 



Maxima and Minima. — The influence of extreme temperatures is 

 more readily demonstrable than the relation of communities to the 

 general range of temperature or to definite means. Excessive heat 

 coagulates the protoplasm. Extreme cold kills the plant by precipita- 

 tion of the proteins. By the formation of ice within the plant, water is 

 withdrawn from the cells, and as a result an irreversible coagulation 

 of the colloidal substances of the protoplasm takes place. Such results 

 from frost are much more frequent than mechanical injury from ice 

 crystals rupturing the tissues. The sociological effect is a reduction in 

 the ability of the species for competition. Long-continued damage 

 from frost or heat or frequent repetition of lesser injury may put one 

 species at a disadvantage and thus favor a less sensitive competitor. 



^ For aquatic communities ordinary measurements have given useful sugges- 

 tions. Next to calcium content, water temperature is one of the most important 

 factors in the composition of aquatic communities. In the Bryum schleicheri- 

 Philonotis seriata flora of the springs of Feldberg in the Black Forest B. schleicheri 

 dominates where the water temperature is 4 to 5°C. in early summer, but P. 

 seriata where it is 7 to 8°C. Otherwise the habitats are alike, and the companion 

 flora identical. 



