468 



THE MONTHLY BULLETIN. 



seeds by irrigation water. He has found that cement ditches not only 

 conserve moisture but eliminate a great amount of waste from weed 

 obstruction and weed seeds. He has placed screen strainers at head- 

 gates and found that a great number of weed seeds were kept off the 

 land. Chemical poisons have also been tried and are proving a great 

 success in keeping ditch banks clean. 



MECHANICAL AGENTS. 



The mechanical agencies are not the most important, though possibly 

 the most interesting from the botanical viewpoint of the means of weed 

 distribution. 



Fig. 96. — Head of Bull thistle, Cirsium lanceolatum, 

 showing the seeds with their parachute-like appendages. 

 This illustrates a type of weed seed which is spread by 

 the wind. (Original.) 



Explosive pods, such as the wild Oxalis, or the shepherd's purse 

 (Capsella hursa-pastoris) , and twisting pods like the wild pea, Erodium, 

 or bur clover, are among the most important of the mechanical methods 

 used by w^eeds. If you have never seen a pea pod hurl its seed you will 

 be surprised at the force it can muster. Hygroscopic movement, as in 

 the awns of the grasses which twist and turn when slightly moistened, 

 thus shoving the seed forward, is another mechanical method. The 



