RESULTS OF MEASUREMENTS. 21 



determined in a single case by numerous and successive readings of 

 the ordinary cog-psy chrome ter. The amount of moisture in the 

 air is determined by this instrument and by the use of tables which 

 show the absolute and relative humidity corresponding to any simul- 

 taneous readings of the wet and dry bulbs of the psychrometer. 



RESULTS OF MEASUREMENTS. 



The pages immediately preceding have been given up to a discus- 

 sion of the methods and principles involved in the measurement of 

 the physical and physiological effects of windbreak. This informa- 

 tion is for the benefit of those who may be somewhat unfamiliar with 

 work of the character undertaken in this study. The next succeeding 

 pages take up the results obtained and the conclusions which may 

 be drawn from a careful analysis of the effects of windbreaks on each 

 factor which influences plant growth. 



I. LIGHT EFFECTS OF SHADING. 



Complete sunlight surveys have been made in the areas adjacent 

 to a number of windbreaks in order to determine the density of the 

 shade cast by different species and the aggregate amount of light re- 

 quired by different windbreaks. The aggregate amount of light used 

 by a windbreak bears a close relation to the amount of damage which 

 results from its presence along the edge of a field. 



The amount of shade cast by any tree in a windbreak, or, in other 

 words, the amount of light used by the tree, depends upon the density 

 of its crown, the extent of its branches, and the orientation of the 

 windbreak. The extent of branching is of most importance in the 

 selection of species to compose the windbreak, or, at least, in the 

 selection of the trees which shall stand next to the field crops. The 

 tolerance of the tree, as expressed by the density of its crown, is also 

 to be considered. 



AMOUNT OF SHADING. 



Table 1 shows the total amount of light cut off by various species 

 in both orientations and, roughly, the location of the heaviest shade 

 as influenced by the position of the greater mass of branches. This 

 latter point is determined by computing the percentage of the total 

 shade that falls within a zone immediately adjacent to the trees, which 

 has a width on either side equal to one-half of the tree height. 



