PHYSIOLOGY 



177 



this, ice first begins to form iu plants at a tenlperature below zero, and only 

 gradually increases in case of a greater reduction of temperature (according to MEZ, 

 to - 6). Such plants as inhabit Arctic and Alpine regions are able to recover 

 from the completely frozen condition. The power which the lower organisms have 

 of withstanding extremely low temperatures is very striking. In PICTET'S investi- 

 gations Diatomaceae were found to sustain a long exposure to a temperature of 

 -200 C. ; MACFADYEN and others have shown that many Bacteria can endure a 

 week's exposure to 250 of cold produced by means of liquid air and hydrogen ( J ). 

 A sudden change of temperature leading to the complete thawing of frozen 



FIG. 173. Trees on the coast of Denmark ; to the left Prunus spinosa, to the right L'rut<ir<iu.i 

 oxycantha, the form of both being similarly modified by the influence of the sea wind. (After 

 WARMING : from SCHIMPER'S Plant-Geography.) 



plants is better borne than repeated transitions from freezing to thaw. A long, 

 keen frost is therefore, as a rule, less injurious to vegetation than less severe 

 frost alternating with frequent partial thaws. 



When the vital powers of reaction of the plant are opposed to 

 purely physical influences, it will naturally depend on their respective 

 strengths which shall become the dominating influence. A centrifugal 

 force a thousand times greater than that of gravity is powerless to 

 influence the reaction of the plant. The degree to which constant 

 and strong currents of air, such as sea-winds, may completely alter the 

 characteristic growth-forms of trees ( 2 ) is illustrated by Fig. 173. 



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