222 



PLANT SOCIOLOGY 



Temperaiure 

 70 60 50 W 



JO''C 



phytes and geophytes avoid them by shifting their vegetative period. 

 Tree seedUngs die out. In the steppes of southern Russia on the 

 Dnieper river, Ramann observed a large number of year-old oak seed- 

 lings with a layer of the stem 1 mm. thick killed at the soil surface. 

 Extreme heat must have caused this damage. 



Decrease of Temperature with Depth of Soil. — The decrease of 

 temperature with increasing depth proceeds the more rapidly the more 



highly the upper soil layers are heated. 

 Even at the highest surface tempera- 

 tures (above 70°C.) the heat is barely 

 noticeable at a depth of 15 cm. (Fig. 

 113). Deep roots are, therefore, 

 never exposed to excessive heat. The 

 excessive surface heat and the wide 

 range of surface temperatures consti- 

 tute a condition that results in the 

 death of many young plants (Fig. 114). 

 While the maxima in the upper- 

 most soil layer by far exceed those 

 of the lowest air layers, the minima 

 fall only slightly below those of the 

 lower layers of air. The denser the 

 soil structure the more readily frost 

 penetrates and the deeper it goes. 

 For this reason loose humus soils are 

 most resistant against frost injury. 



In the colder regions of the earth 

 the season with the lowest tempera- 

 tures coincides with the months dur- 

 ing which an insulating snow blanket 

 covers the soil; hence, there is little 

 frost damage to roots in the coldest chmates. Mats of dead leaves 

 and hchens and mosses are also excellent insulators. 



Nothing is known about damage to roots of wild plants by spring 

 and autumn frosts. In the case of cultivated plants (seedlings of rye, 

 wheat, peas, maize), Zacharowa (1925) has determined that the differ- 

 ence in the cold resistance of the separate parts of the roots is influenced 

 by the reaction of the cell sap. The more resistant cells show a more 

 alkahne reaction than the more sensitive ones. The more alkaline the 

 reaction of the cell sap the more resistant to cold injuries are the roots. 

 This seems due to the fact that the proteins of the protoplasm coagulate 

 at a lower temperature in an alkaline medium. 



Fig. 113. — Decrease of soil tem- 

 perature with depth in the semi- 

 desert of Arizona. {After Sinclair.) 



