SILVICS 3 



best development under a certain set of conditions of soil, 

 climate, etc., called its optimum, and varies whenever found 

 under different conditions. Since topography, exposure, and 

 altitude affect these factors, they must also be taken into account. 

 Near their northern limits of distribution all trees ascend the 

 mountains to a greater height on the southern than on the 

 northern sides, because of warmth requirements. 



Trees derive most of their nourishment from the carbonic 

 acid of the atmosphere, but they depend upon the soil for water 

 and mineral supplies; these can only be taken up when held in 

 solution by the soil. Water is a chief essential of tree growth, 

 part of it being retained in the wood. Greenwood often contains 

 as much as 50 per cent by weight of water. Water in the plant 

 serves as a carrier for the minerals (the salts, oxides, etc.), which, 

 being non- volatile, remain in the plant after the water is given 

 off. The process of taking up minerals in solution through the 

 roots, of depositing these minerals, and of giving off water from 

 the leaves is- called transpiration; this is one of the leading 

 features of plant life. Trees of normal size lose by this process 

 from 10 to 25 gallons of water daily, and it has been estimated 

 that a large oak with 700,000 leaves gives off 244,000 pounds of 

 water in the five months from June to November, or an average 

 of 25 tons a month nearly one ton a day. Transpiration is 

 greater in the daytime than in the night, and leaves exposed to 

 the sun transpire from three to ten times as much water as those 

 that are shaded. There has not yet been developed any very 

 satisfactory theory explaining the movement of water to the 

 top of a high tree. It is known that the minute root hairs at the 

 ends of the roots fasten themselves to the soil particles and with- 

 draw water from them even when they appear dry; and it is 

 probable that the transpiration current, which flows through the 

 wood to the leaves, is forced up by a combination of atmospheric 

 and root pressure aided by osmotic force 1 and capillarity. As 

 has been said, the conifers are more moderate in their demands- 



1 See "A Text Book of Botany," Part I, Sect. II, by Strasburger, Schenck, 

 Noll, and Schimper. 



