1904J Relationship between Weather and Plant Growth. 45 



the last Irost being-, however, earlier (the 6th of May) and not 

 severe. The minimum temperature throughout April was also 

 considerably higher in 1902 than in 1903. So that here again we 

 see the early part of the spring of 1902 to have been a more favor- 

 able season for growth than that of 1903. 



But, while there was very little frost in the spring of 1902, 

 there was one exceptionally severe trost, and that so late as the 

 morning of the roth of May, when the thermometer went 

 down to 21" above zero. This frost did considerable damage 

 to vegetation and must have retarded growth. One Arbor Day 

 we had planted in the same spot at my school round leaved 

 and red osier dogwood shrubs from RockliflFe, and they grew 

 luxuriantly. . This frost of May the loth, 1902, nipped the 

 round-leaved dogwood, withering all its leaves, but spared the 

 red osier growing with it. I noticed that it also spared the alder. 

 But, partly, no doubt, because of their nearness to the ground, it 

 played havoc with many herbs. The early lily and lily-of-the. 

 valley families, in particular, were badly wilted and blackened. I 

 found no CLintonia in bloom that season, owing, I thought, to 

 the frost. 



While in March the minimum daily temperatures ran roughly 

 from 20*^ to 35*^, and in April from 30*^ to 40^, in May and June 

 they ran from 40^ and 45'' respectively to 60". But dividing the 

 months into three periods, it is found that the average minimum 

 temperatures for the three parts are roughly 40^^, 40'^ and ^0° in 

 May 1902, but 40°, t^o" and 50^ in May, 1903 ; and 40*^, 50*^ and 

 50^ in June, 1902, but 50"^, ^o^ and 50*^ in June, 1903 ; showing 

 that the middle part of May and fhe first part of June were much 

 colder in 1902 and that the month of May, 1903, had about the 

 same minimum temperature as the month of June 1902. This 

 reinforces our previous conclusions regarding the phenomenal 

 growth in May, 1903. 



I was struck with the steadiness and evenness of the rise of 

 the minimum as compared with the maximum temperature: a fact 

 due to the constancy with which the earth absorbs heat and the 

 tenacity with which it retains it , as compared With the rapidly, 

 waxing strength of the sun's ever more perpendicular rays, much 

 of whose heat, however, is often intercepted and irradiated by 



