218 PLANT SOCIOLOGY 



Regular gage readings are necessary for the measurement of ground- 

 water level. In various communities of a land-formation complex 

 where the water does not lie exposed, perforated metal pipes (boiler 

 pipes of 5 to 10 cm. diameter are adequate) are firmly sunk in the 

 ground and made as invisible as possible. The readings should be 

 made over a considerable length of time in order to obtain a picture of 

 the seasonal fluctuations of the ground water (cf. Godwin, 1931). 



3. SOIL TEMPERATURE 



The physiological effect of water in the soil rises with increase of 

 temperature. At soil temperatures below freezing, soil water is of no 

 use to vegetation. Plant communities whose root layer lies many 

 months in frozen ground develop late and show a floristic composition 

 much different from that of the surrounding vegetation. In the "cold" 

 moors of the temperate zone there are northern, subalpine, and even 

 occasional alpine species in the midst of the region of beech forests. 

 The subalpine moors of the foothills of the central European Alps, 

 where in May and June at depths of 15 to 20 cm. frozen soil is 

 common, are known as the refuges for relicts of former glacial epochs. 

 Thanks to the local, low soil temperatures, these species have main- 

 tained their place in competition with the surrounding indigenous 

 vegetation. In humus soil the process of thawing is slowest 

 (Wollny, 1898). 



Physiological investigations make it seem probable that the soil 

 temperature acts upon the permeability of the plant tissues and 

 thereby influences the absorption of the inorganic food materials. 

 Cannon (1918) sought to determine experimentally the dependence of 

 growth upon soil temperature of the habitat. He followed root growth 

 of the desert shrub Covillea tridentata at different temperatures and 

 obtained the following average value of hourly elongation : 



The growth optimum of Covillea roots lies at 32°C. It is to be 

 assumed that each species has its own optimum growth temperature. 



The soil temperature is subject to considerable local and temporal 

 fluctuations and is therefore important as a habitat factor. Measure- 

 ments are of value for the comparison of different stations only when 



