26 



WINDBREAKS. 



A corresponding difference in the growth of the trees is also to be ex- 

 pected. The cotton wood windbreaks shown in Plate VI, figures 1 and 

 2, have thoroughly adapted themselves to this difference in the source 

 of light. The east- west row (fig. 2) is almost devoid of lower branches 

 and has a dense mass of foliage near the tops of the trees, while the 

 north-south row has an even distribution of branches from the top to 

 the bottom. The yield tables given on pp. 79-87 show the north- 

 south rows to be proportionately larger producers of timber.. In 

 shading, honey locust again forms an exception to the general rule, 

 since its wide branches shade an unusually large area on the north and 

 south sides. 



424 



tyesr % CAST 



SOUTH NORTH 



DIAGRAM 6. Amount of sunlight cut off by honey locust rows. Expressed in percentages of total 



sunlight on September 19. 



The ground south of windbreaks is, of course, less shaded than that 

 to the north. It might be expected that the branches of trees would 

 extend more toward the sun, but the southerly winds which blow 

 during the growing season do not favor this, and the branches are to 

 some extent bent to the north. That this is because of wind and is 

 not a case of negative heliotropism, as some authors have suggested, 

 is shown by cottonwood. In Kansas the greater growth of branches 

 to the north is very marked; in Nebraska it is less marked, while in 

 Minnesota, where southerly winds are neither prevalent nor especially 

 drying, the deflection is much less. Willow, which is especially sus- 

 ceptible to this influence, shows about the same deflection in Nebraska 

 and Minnesota, and in the latter State quite as much to the east as to 

 the north. An exception is green ash, whose branches are not 

 flexible. 



