HARM DONE BY WEEDS 9 



of food-making from the simple substances which the 

 plant takes in may go on. Some crops can tolerate 

 the absence of a free supply of light better than others, 

 but as a general rule the more light the better. A large 

 crop of weeds tends to restrict the light supply, and has 

 therefore a bad effect on the cultivated crop. The supply 

 of heat to the soil and crop is also restricted, and the 

 free circulation of air is prevented. Ripening corn crops 

 especially suffer in this way from a profusion of weeds, 

 both as standing crops and when stocked to dry. Wollny 

 found that an unweeded soil was colder to a depth of 4 

 inches than a soil kept free from weeds. 



(3) Weeds also absorb from the soil and " transpire," 

 or pass off into the atmosphere, large quantities of 

 moisture which would be of great service to the 

 growing crop. For example, a maize plant has been 

 observed to transpire in the 16 weeks between May 

 22nd and September 4th as much as 36 times its own 

 weight. 1 A large oak tree is also stated to transpire 10 

 to 20 gallons of water in a day ; while barley, beans, 

 and clover were found to transpire, during five months 

 of their growth, over 200 times their dry weight of 

 water. Experiments conducted at the Agricultural 

 Experiment Station of Cornell University showed that 

 during the growth of a 6o-bushel crop of maize the 

 plants pumped from the soil, and transpired into the 

 air through the leaves, upwards of 900 tons of water. 

 A 25-bushel crop of wheat similarly disposed of 500 

 tons of water. Weeds also transpire, and if the ground 

 be covered with weeds it is certain that much of the 

 moisture which would be of value to the crop will be 

 lost in the manner indicated. Weeds are especially 

 harmful in this way in a hot summer, and the loss is 

 most felt by the cultivated crop on light sandy soils. 



1 Fream, Elements of Agriculttire, p. 108. 



