Many plants havo hecorae slowly adapted to a wider heat range.. 

 Most of our cereals are natives of a serai-tropical regions. Corn 

 is "becoming adapted to more northerly/ sections. No part of the 

 earth's surface is entirely destitute of plant life solely "because 

 of a"bsence of heat. In polar region plants carry on life and 

 reproduction during sunless periods at a temperature ranging from 

 18° to 0*. 22 plants out of 2-7 carry on the work of reproduction 

 at that temperature. 



Many plants must acquire means for protection against extremes 

 and sudden ©hanges in temperature. The latter are more injurious 

 than low temperatures, and sudden thawing is detrimental to plant 

 organs. Plants on eastern exposure often suffer from night frosts 

 since they are reached "by the early rays of the sim. 

 Protection against low temperature. 



1. Peculiarities in characteristics of protoplasm. 



2. Changes in characteristics of cell contents. Many conta.in 

 substances resem'bling resin. Ex. Siiow Algae. One plant endured, 

 unprotected, a temperature -46°. 



3. Amount -of Moisture . Much moisture predisposes to little 

 endurance of low temperature. Young twigs suffer most from cold. 

 In polar regions such twigs freeze stiff at night without injury 

 pro'ba'oly owing to peculiarities of protoplasm. Dry seeds can 

 endure many years in Artie regions. 



