150 THE BOOK OF WHEAT 



" drowned. " It also tends to develop the straw indefinitely, 

 and at the expense of the grain. Rains during heading are apt 

 to prevent filling, and are by far the most common cause of 

 blight. In a very wet harvest wheat is apt to germinate be- 

 fore it can be threshed. One-third of the wheat crop of lower 

 Canada was lost in 1855 by the grain germinating in the straw. 



UNFAVORABLE SOIL. The soil texture may be such as to de- 

 prive the roots of the proper air supply. Certain elements, as 

 in the case of alkali, for example, may be present in such 

 abundance that their chemical action upon the wheat plants 

 causes disease. Some of the essential plant foods may be 

 absent, or present in improper proportion. 



All types of disease mentioned thus far arise from physio- 

 logical variations due to abnormal variations in the growth fac- 

 tors, are not transmissible, and consequently never spread 

 from plant to plant or field to field, as do the infectious wheat 

 diseases. 



PLANT INFLUENCES 



Unfavorable Animate Environment. WEEDS Plants out of 

 place are called weeds. They deprive wheat of its nutri- 

 ment and ordinarily give very little in return, except in the 

 case of certain legumes. Weeds once introduced into a region 

 spread rapidly. With runners, rootstocks, running roots and 

 apparatus for throwing seeds, they effect a dispersion of their 

 kind independently of any external agencies. Wind, water and 

 animals are the natural agencies that aid in the dispersion, but 

 rarely carry seeds long distances. Man aids weed migration 

 more than all natural means combined, and consequently its gen- 

 eral direction is in the wake of the progress of cultivation. 

 Commerce in wheat makes some weeds cosmopolites. These 

 plants have a wide range of adaptability, which grows wider 

 under conditions of cultivation. Some seeds, especially those 

 of cockle, have a tendency to approximate the wheat grain on 

 account of selective influences arising from cleaning seed wheat. 

 Those which differ most from wheat are the ones removed. 

 Sowing the remaining ones develops a strain more closely re- 

 sembling the wheat grain. 



Weeds injurious to wheat may be divided into three classes, 

 based on the point of incidence of the damage caused: (1) 



