18 ELEMENTARY FORESTRY. 



have taken from the soil and given off to the air from 500,000 to 

 1,500,000 pounds of water, or from one-quarter to one-half as 

 much as agricultural crops. It has been estimated that the 

 leaves of deciduous trees transpire one-sixth to one-third as 

 much water as an equal surface of water. Large deciduous 

 trees undoubtedly give off as much as a barrel of water a day 

 in dry summer weather. Coniferous trees transpire much less 

 water than most deciduous trees, frequently not over one-sixth 

 as much. 



Mineral Substances are taken up in small quantities and 

 consist mostly of lime, magnesia and potash. They are carried 

 to the leaves, where they are used (perhaps also on their passage 

 through the tree), with a part of the water, in food preparation. 

 The main part of the mineral substances taken up remains as 

 the water transpires in the leaves and young twigs, and is 

 returned to the soU when the leaves are shed, and when the 

 tree is cut and the brush left to decompose and make humus. 



The Soil of Woodlands is Improved from year to year if 

 the leaves and litter are allowed to remain on the ground and 

 fire is kept out, since the mineral matters taken up by the tree 

 are largely returned to the soil in a more soluble form and the 

 amount of humus is increased. For this reason there is little 

 need of alternating woodland crops. 



Almost any soil can furnish a sufficient quantity of mineral 

 substances for the production of a crop of trees, provided it is 

 moist and the leaf mould is not removed. Good soils will con- 

 tinue to furnish mineral matter in sufficient quantity, even if a 

 portion of the leaf mould is carried away. If, however, this 

 removal is continued annually for a long period, any but exceed- 

 ingly fertile soils are likely to become exhausted, just as land 

 on which field crops are grown cannot produce crops forever 

 without manuring. 



The Yearly Round of I/ife in a Tree. In the spring the 

 tree starts into growth and feeds on the plant food stored up 

 the preceding year; the leaves unfold and commence furnishing 

 plant food. These two sources of food push the growth along 

 very rapidly in the spring and early summer. By the first of 

 July the food stored up the previous season is exhausted in 

 many trees, and growth is entirely dependent upon the food fur- 



