42 WEEDS OF FARM LAND 



tuberosum) (Fig. 24) is a variety of tall oat in which the stem 

 becomes greatly swollen between the leaves, giving rise to a 

 string of " knots " with leaves springing from between them. 

 These " knots " are very tenacious of life, and each is capable 

 of developing into an independent plant. It has been sug- 

 gested that this habit of growth is only induced by special 

 conditions of environment, but experiments by L. M. Under- 

 wood l at Rothamsted indicate that the plant is a genuine 

 variety, breeding true to type under whatever conditions it is 

 grown (see p. 90). 



Most of the farm weeds that are at all conspicuous are de- 

 scribed above, but in many others the method of distribution 

 is not sufficiently striking to merit inclusion with the outstand- 

 ing types. Most of these plants are dependent upon seeds for 

 their reproduction, though some spread by their vegetative 

 parts, but the quantity of seed produced and the efficiency of 

 the vegetative reproduction is not sufficient to bring the weeds 

 into prominent notice except under very special circumstances, 

 when from one cause or another a comparatively insignificant 

 weed becomes abundant and demands attention. 



1 Underwood, L. M. (1912), "A Note on Onion Couch," Journ. Ag. S., 

 IV, pp. 270-272. 



