EFFECTS OF FKOST. 35 



What is the optimum amount of oxygen required? 



What is the optimum temperature required ? 



Compare the precocity of unripe and ripe seeds of any plant. 



XXV. EFFECTS OF FROST. 



Wrap up a leaf of Begonia in thin rubber-cloth, to protect it 

 from moisture, and place it in a freezing mixture of powdered 

 ice and salt. After an hour examine the tissues of the leaf with 

 special reference to any mechanical injury which they may have 

 sustained. Having completed this preliminary study, proceed 

 to the examination of any well-developed seedlings, and note in 

 every case (1) the effect produced upon the parts which have 

 been quickly thawed; (2) the effect where thawing has been 

 allowed to go on very slowly. 



Freeze any strong seedlings and after a time thaw them 

 slowly. Place them then under favorable conditions for growth, 

 in order to ascertain whether their vitality has been destroyed. 

 In cases where death of the part or plant ensues, does it appear 

 to come from the freezing or from the thawing? 



