LIGHT IN RELATION TO TREE GROWTH. 



183 



or during a definite period of their life, as, for instance, during 

 the period of flowering and leafing. Thus, the opening of 

 the buds in many trees proceeds much faster when the tree 

 is exposed not only to diffused light, but also to the direct 

 rays of the sun. Therefore, in determining the effect of light 

 on vegetative processes it may be essential to know what portion 

 of the entire light affecting the tree is diffused and what is 

 direct. 



Both direct sunlight and diffused light decrease with increase 

 in latitude, but not in the same proportion. The intensity of 

 direct sunlight decreases much more rapidly with increase in 

 latitude than does the intensity of diffused or skylight. This 

 is well brought out in Table I., computed on the basis of 

 measurements taken by Bunsen and Roscoe, which gives the 

 relative chemical intensities of the radiation received directly 

 from the sun and from the sky upon a horizontal surface during 

 a whole day at the spring equinox. It is of considerable interest 

 as showing what kind of light is mostly available for tree growth 

 at various latitudes. 



Table I. — Light intensity of direct sunlight and of diffused 

 light at various latitudes} 



Diffused light decreases with increase in altitude ; direct 

 sunlight, on the contrary, increases. 



Overhead^ Side and Reflected Light. — Besides direct and 

 diffused light, light may be distinguished in accordance with the 

 direction from which it comes and the effect it has upon the 



^ From Handbook of Climatology, by Dr Julius liann, p. Ii6. 



