Exposure Efect on White Pine Seedlings 



167 



Check 



93 



84 



87 



Discussion of Results. — The nine check series of 175 trees each, 

 or 1,575 (diminished by 125 where Lophodermium was found), 

 show a survival of 80 to 93 per cent. As long as the survival of 

 80 per cent or more of the exposed trees, therefore, is shown the 

 exposure cannot be regarded as serious. 



A 10- to 40-minute exposure, whether occurring in the fall, in 

 the spring, or in the fall and again in the spring, is not serious 

 except on a windy day with low relative humidity. This supports 

 the result of the earlier experiments. (F. Q. Vol. XII, pp. 31-33.) 



A comparison of fall exposure with spring exposure fails to 

 show an entirely consistent difiference, although the spring ex- 

 posure in general seems to show the greater loss. In the absence 

 of a comparison of relative humidity on the days of exposure 

 between fall and spring definite conclusions should hardly be 

 drawn as to the season factor alone. Since the injury to the 

 seedling due to exposure comes from both evaporation and trans- 

 piration, spring exposure late enough for active transpiration, 

 Vt^ould logically seem the more injurious. 



The effect of the air movement seems to be marked, as the 

 clear windy day exposures show a much greater loss, even though 

 the relative humidity (73% to 91%) is higher than on the calm 



