682 



WEED FLORA OF IOWA 



Fig 529 Fig. 530 



Fig. 529. Powdery Mildew of grasses (Erysiphe graminis). This is common on 

 Wild Barley, occurs also on Wheat, a, mycelium and erect conidiophores ; 

 b, c, conidia. 



Fig. 530. Cocklebur (Xanthium canadense) . There is a widespread belief that 

 Cocklebur and other weeds may excrete a substance which is injurious to 

 other crops. 



clover would not do as well in soil of this character, nor did the 

 clover seed germinate as well, as in check soils. However, this 

 subject has been investigated but little. Then there is the larger 

 question of the actual antagonism between roots in occupied soils 

 that should be occupied by the roots of agricultural plants. 



11. WEEDS STOP DRAINS. 



Drain tiles are frequently filled with a growth of the roots of 

 weeds, causing stoppage and often much expense in removing the 

 difficulty. 



