Killing of Plant Tissue by Low Temperature 



217 



Table 26. Showing Sap Density of Apple and Peach Twigs 

 Throughout the Year as Measured by the Freezing 



Point Depression. 



*Only one depression taken in December. 



It will be seen that while the sap density of the cortex of winter 

 twigs is much greater than that of early summer twigs, yet it is not 

 appreciably greater than that of twigs in late September and October, 

 when the tissue is still considerably more tender than in December 

 and January. Some may suspect that the low sap density of the 

 early summer twigs may be due to their young and somewhat succu- 

 lent condition. It may be said, however, that the density of the 

 cortex of these young twigs is generally greater than that of any 

 other tree tissue except the leaves. (Data to be published in another 

 bulletin ) . 1 1 would seem certain then that while a part of the increased 

 hardiness of tree tissue in winter may possibly be accounted for by 

 the greater sap density, not all of it can; certainly not the greater 

 hardiness of December tissue over that of October. 



I n the case of plants killing at as low temperature as those at which 

 winter twigs kill, it seems possible that the sap solute, if it remains 

 in solution, may tend to keep a small amount of water unfrozen 

 and thus protect the protoplasm to some extent. If this should 

 be true, the eutectic point of the sap solute would play a very import- 

 ant part in determining the amount iA killing. Some efTorts were 

 made to determine whether or not there may be changes in the sap 

 solute as winter comes on that give it a lower eutectic point. Just 

 at the time of leaf fall or slightly before, twigs had the cortex scraped 



