21' 



the dense, thick-walled summer wood cells make stronger wood than the 

 thin-walled spring cells, and in a year of rapid growth the summer wood 

 formed in some species exceeds in amount that formed in years of slower 

 growth. 



The food necessary for a tree to increase its solid substance is obtain- 

 ed from the soil and from the air. Most of this food substance is 

 formed in the green parts of the plant the foliage, in the presence of 

 light and air by the union of water with the carbon derived from the car- 

 bonic acid of the air. From the soil, water is constantly being lifted by the 

 tree up into its foliage ; from which a part passes off into the air as vapor, 

 the amount of transpiration varying with the climatic conditions, water 

 supply, season and species. While in a vigorously-growing tree there is 

 from 40 to 65 per cent of water, the amount given off from its leaves in a 

 season is many times greater than that retained, but trees require from one- 

 half to one-quarter of the water which agricultural crops need. 



Mineral substances are taken up only in very small amounts, and 

 mostly of the commoner kinds, such as lime, potash, magnesium and nitro- 

 gen. Hence, wood crops do not exhaust the soil of its minerals, and even 

 improve its fertility, as the greater part of the minerals are returned to the 

 soil in more soluble form by the annual fall of the leaves and the small 

 brush, in which the minerals are most abundant, and which decompose and 

 form a rich humus layer on the surface of the soil. 



As the soil moisture is the greatest requisite for tree growth, its con- 

 servation and distribution is most important. No tree grows to best advan- 

 tage in very wet or dry soil, although some species endure and appear 

 thrifty in such unfavorable situations. The soil most suited for all trees is 

 a moderately but evenly moist soil, porous and well drained, but capable of 

 conducting water up within reach of the roots of plants. 



For the conservation of the soil moisture, the forest grower cannot re- 

 ly on the methods of the agriculturist, which are usually impracticable and 

 too expensive. He can only employ such methods as shading and mulch- 

 ing the soil shading by close planting, and by maintaining the crown 

 cover dense through the life of the crop to protect the soil from sun and 

 wind ; mulching, by the annual fall of twigs and leaves, which remain and 

 decay, forming a rich mold, increasing the absorption and retentiveness of 

 the soil, and retarding the evaporation of moisture and the run-off from the 

 surface. 



While with a moderate and even supply of moisture all trees thrive 

 best, some like the Conifers, and especially the Pines, endure drier soils, 

 and others like the Bald Cypress, excessive moisture. This adaptation, how- 

 ever, is modified in different regions by drier or more humid climate. 



