Autumnal Colors: General Views. 79 



— 23 inches of mercury, a difference of 69- inches.' In the poplar- 

 leaved white birch, it was from -|- 35 to — 17, in the body of the 

 tree, and from -f. 33.6 to — 20.2 in the root. In the yellow birch 

 (Betidaexcdsa), it ranged from -|-65.5 to — 18.5 ; in the canoe-birch, 

 from -[- 59 to — 7 ; and in the grape-vine, from -|- 74 to 12.7 inches. 

 Where one gauge was placed near the root of a tree, and another 

 near the top, they indicated a difference due to the hydrostatic 

 pressure of a column of water equal in length to the distance be- 

 tween the two levels. Trials were made upon the butternut, iron- 

 wood, apple tree, etc., with corresponding results, but less in degree. 

 These experinaents were made before the leaves' had opened, and 

 the force of suction could not therefore be ascribed to evaporation 

 from their surfaces. 



Autumnal Colors. 



283. The coloring of autumnal leaves appears to be due to the 

 formation of organic acids from the absorption of oxygen, and 

 caused by a ripening process, similar to that which colors ripening 

 fruits. It is not the effect of frost, as many people believe, but 

 may be hastened by the cool nights alternating with warm days, 

 that often occur in autumn. The autumnal coloring of European 

 forests is sometimes bright, but never as brilliant as in our Northern 

 States "and in Canada. Its prevailing colors are yellow, shading off 

 into tints of pale orange and reddish brown, while in our northern 

 forests it is often the brightest scarlet and orange, a rich golden yel- 

 low, or an intense purple, but all passing gradually into a nearly 

 uniform shade of brown. 



CHAPTER VIII. 



GENERAL VIEWS IN EEGARD TO FORESTEY. 



Of the Investment of Labor and Capital in Forest-tree Planting — Ques- 

 tions of Profit. 



284. As a general rule, the most profitable lands for planting, at pres- 

 ent prices, are not those that would yield the best farm crops, although 

 the best lands will always produce the best growth of forest trees. 

 Upon such lands, forest trees would be sure to thrive, but the profits 



' In these statements, the sign (+) is used to denote the outward, and ( — ") 

 the inward piessure. 



