INJURIOUSNESS OF WEEDS 



681 



Fig. 528. "White Rust (.AWugo Candida). 1. Inflorescence of Shepherd's Purse 

 with fungus. 2. Mycelium with haustoria (h) x 390. 3. Conidiophores 

 and conidia (spores) in chains x 400. i and 5. Formation of zoospores in 

 conidia x 400. 6. Germinating zoospore. 7. Oogonium (o) and anther- 

 Idium (a) attached, mycelium shown below. 8. Oospore with thick wall. , 

 9. Germinating oospore forming a zoosporangium. 10. Zoospore 7-10 x 400. 

 (After DeBary.) 



8. WEEDS PREVENT THE PROPER CULTIVATION OF SOIL. 



Weeds of certain types, particularly the perennial weeds, often 

 make it extremely difficult to cultivate a soil. This is especially 

 true of weeds like quack grass, which when present in the soil 

 make it difficult not only to cultivate, but to plow and harrow, and 

 also make the operation quite expensive. 



9. WEEDS MAY CAUSE CONDITIONS WHICH BREED DISEASES. 



Dr. Evans says that tall weeds contribute to the breeding places 

 of mosquitos, and he thinks that in this way the mosquitos will 

 lead to malaria. It is a well-known fact that the pollen from 

 weeds like ragweed and goldenrod cause hay fever and for this 

 reason these weeds should always be removed. 



10. WEEDS MAY POISON THE SOIL. 



There has long been a popular impression that weeds excrete 

 poisonous substances which render the soil unfit for a succeeding 

 crop. In regions where cockleburs are common, it is quite difficult 

 to get a good stand of clover. Mr. E. B. Watson found that 



