12 LOCALITY AND FOREST VEGETATION. 



and growth of plants than the degree of fertiUty of the soil ; 

 hence it demands the forester's special attention. Generally 

 speaking, the climate of a locality depends on : — 



(1.) Latitude and longtitude, or geographical position. 



(2.) Elevation ahove the level of the sea. 



(3.) Aspect and gradient. 



(4.) Shape of the surface and condition of surroundings. 



Each of these affects the heat, light and humidity of a 

 locality, which are the agencies determining the commence- 

 ment and course of the annual phenomena of vegetation. 



1. IlcaL 



Heat affects plant life in various ways. In the first place, 

 it is necessary for transpiration h}^ the plants and evaporation 

 from the surface of the earth ; and secondly, it governs the 

 movement of the air, which produces a thorough mixture of 

 its different ingredients, as well as that of warm and cold, dry 

 and moist, clear and hazy air. 



The heat required hy plants for transpiration and growth 

 must he supplied to them hy the atmosphere, either directly 

 or througli the soil. If these are themselves deficient in heat, 

 transpiration must cease as soon as the plant has expended 

 the store of heat which it contains. The latter, however, fre- 

 quently does not take place until serious damage has l)een done 

 to the plant, in other words, the temperature of the plant may 

 he so far reduced that the freezing point is reached, although 

 the temperature of the surrounding air is still several degrees 

 above that point. 



The only important source of atmospheric heat is the sun ; 

 hence the temperature of a locality depends in the first place 

 on its latiiudc. The mean annual temperature decreases with 

 the distance from the equator because the sun's rays strike the 

 earth more obli(juely in proceeding iiorth oi* south from the 

 equator towards the poles. In the centre of Europe and 

 in the vicinity of the 50th degree of latitude, the tempera- 

 ture decreases one degree for ul>uut every Ol miles, on 



