EFFECTS OF SUN-LIGHT ON VEGETATION. 263 



processes, and before the earth, still enveloped in its snowy 

 covering, has felt the influence of returning spring. This 

 is evidently mainly or wholly due to the sun's light 

 direct or reflected ; and perhaps its rays as reflected from 

 the pure and glassy surface of the snow, after the days 

 have increased considerably in length, may have the same 

 powerful effect on the forest that, according to Professor 

 Forbes, they have on the black-bulb thermometer. For 

 some time after the trees have begun to thaw by clay, they 

 freeze again in the night ; and in more southern localities, 

 where the sugar-maple grows, the sugar makers are well 

 acquainted with the fact that a hard frost arrests the 

 flow of the sap in the night. Should a hot day, however, 

 follow such an occurrence, the flow is more abundant 

 than ever, the short rest seemingly increasing the irrita- 

 bility of the organs by which the sap is eliminated and 

 circulated.* 



* As I was revising this sheet, Sir "William Hooker favoured me 

 with an extract from the journal of Mr. Berthold Seeman, botanist of 

 the Herald, part of which follows : — " During our stay at Port 

 Clarence, in September, 1850, 1 made several experiments to ascertain 

 the depth to which the thaw penetrated the soil : the result varied ; 

 in some places it did not descend above two feet into the earth, while 

 in sandy places the ground was free from frost to the depth of four 

 or five feet. The season was much colder than in 1849, the sea more 

 loaded with ice, and the terrestrial vegetation less vigorous.' 



s 4 



