WINTER INJURY 



277 



outside temperature around — 10°C., the leaves on the upper part of 

 the stem were very much wilted, because of the interference with sap 

 conduction in the portion of the stem outside. With rising temperature, 

 however, they recovered. ^^ 



10 II 12 13 14- 15 16 

 February 



Fig. 27. — Temperature fluctuations in a tree trunk. (After Squires^^) 



Table 22. — Tree and Air Temperatures 



(After Squires''-^^) 



Januarv 



February 



Air, degrees 

 Centigrade 



Tree, degrees 

 Centigrade 



Air, degrees 

 Centigrade 



Tree, degrees 

 Centigrade 



6 to 7 a.m 

 12 to 1 p.m 



6 to 7 p.m 

 En tiro dav 



10.84 

 6.60 

 9.20 



s.ss 



-9.37 

 -8.90 

 -7.50 



11.93 

 4.35 

 8.65 

 8.31 



-10.46 

 -7.55 



- 7.00 



- 8.34 



These considerations make it evident that prolonged cold weather 

 must interfere materially with sap conduction while at the same time the 

 conditions accompanying extreme cold are the very conditions which 

 favor greater drjnng out. It should be considered, too, that the conduc- 

 tive regions in the tree are near the outside of the stem. Many cases of 

 twig killing must, therefore, be considered as due to drought, short, 

 perhaps, but intense in localized areas. Possibly the greater suscepti- 

 bility of twigs to this injury is not due entirely to failure in conduction 



