SILVICS 3 



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 lo 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 ^ and capillarity. As 

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

 on water than the broad-leaved genera. This is the reason why 

 sand plains, such as those of Cape Cod, can support the pine. 

 The water table is always found to be farther removed from the 

 surface of the ground under forest than outside it or under a cut- 

 over forest; under old forests it is lower than under young 

 stands. Seedhngs spring up in a forest when the roots of the 

 adjoining trees are cut through, thus allowing the water table to 

 come nearer the surface. From the illustration of the oak 

 mentioned above it will be readily realized, in dry seasons 

 especially, that the amount of water available per tree might 

 well be the controlling factor influencing growth or life itself. 

 Forests transpire more moisture than other vegetation under 

 the same conditions. Any deficit caused by excess of transpira- 

 tion over precipitation is necessarily made up by water from the 



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

 Noll, and Schimper. 



