94 A TEXTBOOK OF BOTANY [Ch. Ill, 12 



Thus with our Maples, the partial splitting away of a branch, 

 an injury to the bark, or infection by disease, will often pro- 

 duce the red coloration in the leaves of the injured branch 

 while the remainder of the tree is still green. Further, a 

 bright climate is essential to the best coloration, partly be- 

 cause bright light produces a quicker and fuller fading of 

 the chlorophyll, and therefore a better exposure of the xan- 

 thophyll, and partly because the brilliancy of* erythrophyll 

 formation is directly proportional to the brightness of the 

 light. It is because brighc days and frost go together that 

 the latter is commonly credited with more than its due 

 in the process. The conditions of the preceding summer, 

 whether dry or wet, play also some minor part, through 

 influence on leaf vitality. In general, other conditions 

 being equal , the brightness of autumn coloration in any given 

 region is proportional to the clearness of its autumn climate, 

 while its brightness in any given season is proportional to 

 the clearness that year. This importance of light explains 

 why the color is more vivid in climates like that of New 

 England, where the autumnal skies are prevailingly bright, 

 than it is in old England, where autumn is a season of mois- 

 ture and cloud. Finest of all is the coloration in places where 

 the summer ends abruptly, the autumn is bright, and the 

 frosts come early, as occurs in Eastern Canada, where some 

 of us think it is the best in the world. 



12. The Economics, and Treatment in Cultivation, 

 of Leaves 



All cultivation of plants depends for its success upon con- 

 formity to their physiological peculiarities. It is true, 

 gardeners and farmers have not had in the past any scien- 

 tific knowledge of these matters, but through centuries of 

 experience, consisting in observation and trial and the passing 

 along of the results, they have reached conclusions nearly 

 enough correct for all practical purposes. We consider now 

 the practice of plant cultivation with respect to leaves. 



