MOISTURE. 21 



it thrives but indifferently in continental situations with a 

 dry air. The Scotch pine, on the other hand, appears over 

 extensive tracts in continental dry climates, and at the 

 same time flourishes in the moist insular climate of Great 

 Britain. 



Although further investigation is necessary to show whether, 

 or to what degree, trees take up moisture through their organs 

 above ground, the atmospheric moisture is of the highest 

 importance, for the following two reasons : — 



(a.) It governs the degree of transpiration from the leaves 

 of the trees. 



(b.) It supplies the soil with water, whence it is taken up by 

 the roots of the trees. 



Apart from the temperature, the degree of transpiration 

 depends on the degree of saturation of the air ; hence 

 relatively dry air causes rapid transpiration, while the latter 

 ceases as soon as the air becomes saturated. The degree 

 of transpiration, in its turn, governs the rapidity with which 

 fresh water laden with raw materials is taken up by the 

 roots. 



The soil receives water from the atmosphere in the shape of 

 precipitations, as dew, rain, snow, hail, and a certain amount 

 by means of its hygroscopic nature. The quantities differ 

 enormously in different climates and localities, from almost 

 nothing to over 500 inches a year. Precipitation cannot take 

 place unless the air becomes saturated. The phenomena 

 which favour an increase in the relative humidity, and thereby 

 induce saturation, are active evaporation and a reduction of 

 temperature. On the one hand, evaporation causes a reduc- 

 tion of temperature, and on the other a falling temperature 

 reduces the degree of evaporation ; the result is, that saturation 

 and precipitation occur only locally. 



As a general rule a low temperature means a high degree of 

 relative humidity ; hence the latter is greater in winter than 

 in summer; greater at high elevations than in low lands; 

 greater in the vicinity of extensive sheets of water than in 



