The plant tissue may have a temperature different from the 

 air temperature when the sun is shining brightly because the 

 dark color of the tissue absorbs the sunlight. We have thus 

 known the trunk of a tree on the side next to the sun to show 

 a temperature of more than 20 degrees above that of the other 

 side, and the dark twigs may on this account show a tempera- 

 ture of 15 or 20 degrees above the air temperature. When the 

 sun is not shining, however, we are generally safe in assuming 

 that the tree tissue, except in case of very large trunks, is at a 

 temperature very near to that of the air. At a few degrees 

 below the freezing point, therefore, we will generally find ice 

 throughout the tissue. 



We also knew that in some way this ice formation is as- 

 sociated with the killing of the tissue; at least, tissues that kill 

 at rather high temperature will not be killed unless ice is 

 formed. Thus it is known that water may reach a temperature 

 several degrees below freezing without ice formation. The 

 juices in the plant may sometimes do the same thing. If, 

 when the plant tissue is thus supercooled, it can be warmed 

 back up to above the freezing point without ice formation, no 

 injury results, even when the temperature to which it had 

 fallen was low enough to kill it if ice had formed. This, of 

 course, does not mean that ice formation always kills. More of 

 the water must be in the form of ice to kill some plants than 

 others. In case of the wood of our trees, nearly all the water 

 is frozen out before killing takes place. We, of course, do not 

 know whether the killing results from the taking of the water 

 away from the protoplasm in ice formation or whether it is 

 the extent of the ice mass crushing or otherwise injuring the 

 protoplasm. 



It has often been said that if the tissue thaws slowly it may 

 be uninjured, when it might be killed by rapid thawing! This is 

 true in the case of ripe apples and pears, provided the tem- 

 perature has not been too low. With these fruits in storage, if 

 it becomes cold enough to kill them partially, the injury may 

 sometimes be avoided by keeping them where they will thaw 

 very slowly. This does not seem to be true of any other 

 tissues; thus, in case of the wood of our trees and certainly in 



