460 Plant Physiology 



It is obvious that the use of chemical sprays for weed 

 eradication in the field is dependent upon the resistance of 

 the crop as compared with the weed and to the penetration 

 of the chemicals employed. This method has been found 

 especially applicable in growing cereal crops, grasses, flax, 

 and peas. 1 



The plants which are killed are those whose surfaces are 

 easily wet by the spray, but there are some plants, the 

 common plantain (Plantago major), for example, which, 

 although wet, is almost unaffected. Those which are not 

 wet generally possess smooth glaucous leaves, or are pro- 

 vided with a waxy bloom. In any plant the succulent 

 or rapidly growing portions are more easily killed. Thus 

 it follows that this means of eradication may be generally 

 employed for plants with an indefinite habit of growth. 



LABORATORY WORK 



Toxic action. Determine the limiting' concentrations of 

 CuSO 4 and H 2 SO 4 for inhibition and growth of roots of corn 

 and peas. Use the tumbler-culture methods employed in the 

 study of mineral nutrients, or, if observations cover only a 

 short interval of time, the germinating seeds may be pinned to 

 the lower surfaces of corks, covering the vessels employed, the 

 roots projecting into the solutions. Make decinormal stock 

 solutions of the toxic agents. With the CuSC>4 employ at least 



1 From a considerable number of experiments, it has been found that 

 such plants as the following are more or less readily killed: bindweed, 

 Canada thistle, dock, great ragweed, lamb's quarters, mustard or char- 

 lock, orange hawkweed, sow thistle, wild buckwheat, and wild radish. 

 Weeds which have not been successfully combated without injury to the 

 growing crop are such as bentgrass, bull-thistle, couch-grass, horse-tail, 

 pigweed, and others. With iron sulfate Bolley has been able to hold 

 the dandelion in check ; but on account of the perennial root, this plant 

 is one of the most difficult to eradicate. 



