168 Forestry Quarterly 



days. The fall exposure here up to 40 minutes or even one hour 

 again shows no marked loss, though the spring exposure of 20 

 minutes, and the double exposure (fall and spring) of 10 minutes 

 even, each shov^ a marked decrease in vitality. This supports 

 entirely the somewhat indefinite, but nevertheless general, experi- 

 ence among nurserymen and silviculturists that the windy day 

 is the dangerous day for the exposure of coniferous stock. 



As between the calm cloudy day and the calm clear day the case 

 is not so plain. The clear-day spring exposure was made with a 

 relative humidity of 39 to 48 per cent, while the cloudy day had a 

 relative humidity of 63 to 77 per cent. Yet the cloudy day seems 

 to show the greater loss, although the results here are again not 

 quite consistent. Mr. Harbeson suggests that the transpiration 

 loss on the cloudy day may account for this, since in the sun the 

 stomata are apt to close and secure more effective insulation for 

 the needles. 



The heeling in over winter does not seem to weaken plants of 

 this character, as is shown by the high survival percentage among 

 the check trees, which is just as high as with the spring raised 

 seedlings used a year ago. 



As suggested in the article last year, the ill effects of exposure 

 do not manifest themselves only through the total loss of plants, 

 but even though they survive, their vigor may be impaired. Trees 

 exposed 40 minutes and less showed an average current growth of 

 2^ inches, while those surviving the longer exposures showed 

 less than half this growth or about lyi inches. 



The following notes on a rough attempt at the same problem 

 are submitted by Professor O. L. Sponsler, of Michigan 

 University : 



About 200 seedlings, three-year-old White pine were exposed. 

 It was observed that the evaporation during the first 20 minutes 

 was much greater than later on, the rate of loss decreasing rapidly 

 up to an hour, when exposure ceased and the plants were planted 

 and left without attention until fall ; then the following conditions 

 were noted : 



