BURNS: WEATHER CONDITIONS AND PLANT DEVELOPMENT 121 



no means as valuable as those gathered by recording instruments 

 during long periods of time. These latter data are valuable, however, 

 primarily because they cover critical periods during which the environ- 

 mental conditions are most severe for plant development. The limit- 

 ing factor is not the average for the long period, but the maximum or 

 minimum for any factor or group of factors during certain critical 

 short periods of the longer season under consideration. This may be 

 made more clear by illustration. The efTect of shade on the develop- 

 ment of white pine seedlings was under study. Lath shades known 

 as "full-shade" and "half-shade" were used. It was found that 

 germination took place sooner and that larger numbers of seedlings 

 were produced in the "no-shade" bed than in those partly or fully 

 shaded. The temperature of the soil was the controlling factor. 

 The average soil temperatures computed from readings recorded every 

 two hours with a Friez machine for the 24 days during the germination 

 period were "full-shade" 47° F., "half-shade" 46.8° P., "no-shade" 

 49° F. These differences are too slight to have been responsible 

 for the observed differences in germination. If we look at averages 

 only and shut our eyes to the daily fluctuations we would conclude 

 that soil temperature was not the controlling factor. When, however, 

 the records were examined for extremes it was found that on certain 

 days temperature variations occurred of as much as 20° F. as between 

 the soils of the different beds, the soil of the "no-shade" bed reaching 

 73° F. It is easy to believe that such differences may constitute a 

 controlling factor, in view of the fact that Atterberg has shown that 

 this temperature is about the optimum for germination of these seeds. 

 One evening the nursery foreman reported that every seedling in 

 our nursery was dead. Examination showed that the white pine 

 leaves which the day before had been a beautiful green were brown 

 and apparently dead. A closer study showed that the ends of the 

 leaves including about one third of the leaf were dead. However, 

 very little of this leaf browning occurred in another nursery located 

 near a river bank, protected from the prevailing wind and on a richer 

 soil. In the upper nursery where the browning occurred we had a 

 number of recording instruments but unfortunately none in the lower 

 nursery. The records show: that three slight showers and one 

 heavy rain (1.28 inches) had fallen just previous to the appearance of 

 the trouble; that for five days the sun had shown from nine to twelve 

 hours daily; that a very heavy wind blew for three days before, 

 especially while the sun was shining; that the humidity dropped 

 daily below 50 percent, one day reaching 35 percent; that the air 

 temperature was usually below 75° 1'.; that slightly protected areas 

 in the upper nursery showed less damage than did the rest of the 

 nursery. 



