Plants as Affected by Excessive Water. 127 
217. Methods of Preventing Injury by Frost. Any 
nonconducting material, lying between the earth and space, 
whether spread directly upon the earth or at a considerable 
height above it, acts as a blanket to intercept the radiating 
heat, and thus prevents in a measure the cooling of objects 
beneath it. For this reason, straw, muslin, or other noncon- 
ducting material, spread over plants, protects them from 
frost. 
But while it is easy to protect a few plants from frost by 
covering them directly, it is much more difficult to protect 
large plantations in this manner. Considerable plantings 
of the strawberry have been successfully protected from 
frost by covering the rows in the evening with straw or 
marsh hay, and where these materials are convenient, the 
work may often be cheaply and quickly performed. | 
Attempts to prevent frost by the heat of fires, or by burn- 
ing material that produces much smoke or vapor, have not 
been sufficiently successful to commend these methods for 
general application. 
Section Il. Tor PLAnt As AFFECTED By UNFAVORABLE 
WATER SUPPLY 
A— BY EXCESSIVE WATER 
218. Excessive Water in the Soil Destroys the 
Roots of plants. We saw (90), that oxygen is necessary to 
the life of roots. When the soil cavities are filled with 
water, the roots are soon deprived of oxygen, because the 
little oxygen contained in the water is soon exhausted (94). 
Smothering and decay of the roots follow. Seeds planted 
under such conditions usually fail. The soil water that is 
most useful to land plants is that which remains attached to 
