86 LIVING PLANTS 



are killed by the winter, and the evergreens 



which withstand it without any great altera- 



Varying tions in outward form, stand the deciduous 



reaction trees, which cast their leaves, and herbaceous 



to cold plants with thickened underground stems, 



w^hich withdraw the living substance from 



the leaves and stems to the underground 



structures, leaving the entire shoot to perish 



in the winter storms. 



The degree of cold necessary to ensure the 

 death of any species depends entirely upon the 

 specific constitution of the protoplasm and 

 the stage of development, or stage of activity 

 of the organism at the time it is subjected to 

 the low temperature. 



According to numerous tests made during 

 the last half century, it has been found that 

 many delicately leaved, rapidly growing spe- 

 cies are killed by a temperature above the 

 freezing point, others native to the Arctic zone 

 are not injured when the air and the soil in 

 which they grow fall to seventy degrees centi- 

 grade below the freezing point. Well matured 

 and air dried seeds in the resting stage have 

 been subjected without injury for prolonged 

 periods to the extremest low temperatures 

 that can be produced in the laboratory. In 

 one series of experiments seeds were immersed 

 in liquid air at a temperature of about two 

 hundred degrees centigrade for several hours 



