WEEDS. 295 



injurious to cultivated plants. Wild barley (Hordeum 

 murinum) causes much loss to stock owners in the 

 west by its pestiferous awned seeds. 



Weeds restrict the circulation of the air as well as 

 taking plant food from it. The amount of carbonic 

 acid which comes in contact with the cultivated crops 

 in a given time is thereby restricted. Too little, 

 however, is known of air chemistry and physics as 

 related to plant gi'owth to make it possible to hazard 

 an opinion upon the effect of this on the growth of 

 the crop. 



The amount of fertility which a crop of weeds 

 takes from the soil may be supplied to the land and 

 still the weeds will be injurious. Some crops are not 

 injured by shading the ground, such as potatoes, 

 which are raised in some localities by mulching with 

 straw. It would seem that the space which the weeds 

 occupy in a corn-field would not be injurious to corn, 

 yet weeds must be removed in order to get a full crop. 

 Until there, is further evidence it must be concluded, 

 therefore, that the most important injury that weeds 

 do is in exhausting the water from the soil. 



Kinds which are Injurious. — No one can tell 

 with absolute certainty whether a weed will become 

 troublesome in a given locality. Not more than one 

 in twenty of our bad weeds is a native plant. Most 

 of the foreign plants which have become troublesome 

 here are of little importance in their native places. 

 Plants indigenous to this country and not usually 

 troublesome here have become great pests when intro- 

 duced elsewhere. Evening primrose and watercress 

 are examoles. 



The pernicious character of weeds varies in different 



