WINTER INJURY 281 



however, the frost-killed crops, no longer growing, ceased to draw on the 

 moisture supply while the frost-resistant crops kept the moisture content 

 low. When it is recalled that Emerson's earlier work showed 19 dead 

 trees and none uninjured out of 25 in soil with 15.2 per cent, moisture, as 

 here under frost-resistant crops, while soil with 19.8 per cent., the nearest 

 figure to that of the soil under frost-killed crops, showed three dead and 

 12 uninjured, the importance of this difference is evident. The graph 

 for the soil with no cover crop shows a somewhat higher moisture content 

 in December than either class of cover crops but it also shows a high 

 moisture content in September and October, suggesting a prolonged 

 growing season and poor maturity in the tops. This is what actually 

 occurred. Emerson's work emphasizes the importance of a water supply 

 after maturity is attained. 



A plentiful supply of available water is an important factor deter- 

 mining the recovery of plant tissues from the effects of low temperatures. 

 Pantanelli has shown that the activity of the roots is of great importance 

 in determining the recuperative power of the plant after the aerial parts 

 have been exposed to low temperatures and that all those factors that 

 reduce the absorbing capacity of the roots, such as insufficient aeration, 

 salinity, alkalinity and the presence of toxic substances reduce the 

 recuperative power of the plant. 



Windbreaks. — The relation to the orchard of shelter belts composed of 

 hardy trees and shrubs has been the subject of much discussion, of some 

 observation but of little precise study. Variations in local conditions 

 of exposure to prevailing winds and in the character of these prevailing 

 winds, as well as the topography of the orchards themselves, pre- 

 clude the possibility of windbreaks being universally beneficial or injurious. 

 Their efficacy, when properly placed, in cutting down the windfall 

 loss from summer storms, is not a matter for discussion here. In 

 the Michigan and New York fruit sections much of the advantage 

 claimed for them is the protection they afford from those types of 

 winter injury that are associated with drying out and they are set usually 

 on a northern boundary of the orchard. In the north central states 

 windbreaks seem to be planted more as protection against the hot drying 

 winds of summer; hence, they are generally set on southerly boundaries. 



Ejfect of Wind Velocity. — Of quantitative data on windbreak effects, 

 little is available. The increased snow deposit in places sheltered from 

 the full sweep of the wind is a matter of common observation. After the 

 snow has fallen the windbreak acts to preserve it from evaporation by 

 protecting it from the full force of the wind. Fernow^^ states that 

 snow evaporates ten times as fast in warm wind (velocity not stated) 

 as in calm air. Provided the snow accumulation is not great enough 

 to injure the tops of young trees this effect must be beneficial since 

 data to be introduced show the great power of snow in protecting roots 



