XVII. MISCELLANEOUS PRODUCTS USED IN AGRICULTURE 363 



dissolved. The dressing is usually applied about 24 hours before 

 sowing and the grain is thoroughly mixed and incorporated with the 

 solution. Each grain of wheat becomes coated with the liquid, which 

 on evaporation, leaves a thin film of the salt. The spores of the fungi 

 are thus destroyed, but the copper is converted into insoluble com- 

 pounds SOOQ after the seed is sown and before germination of the 

 wheat embryo commences. The corn, therefore, is not injured, though 

 it would probably be killed outright if it were not for the action of 

 the constituents of the soil (probably mainly the calcium carbonate) 

 upon the copper sulphate. In America, the grain is soaked in a solu- 

 tion of 1 Ib. copper sulphate in 24 gallons of water, for twelve hours 

 and thea for five minutes in lime water. 1 Dressings of copper sul- 

 piate are also now recommended for barley and oats, for preventing 

 smut. 



Copper sulphate is also employed in solution for spraying plants, 

 with the object of preventing fungoid diseases. For this purpose 

 a solution containing about 0'5 per cent of the salt is usually employed. 

 Stronger solutions would be apt to injure the foliage of certain plants. 



Another use of copper sulphate is in the destruction of cruciferous 

 weeds in cereal crops, e,g., charlock in barley or oats. This is ef- 

 fected by spraying the field when the charlock plants are still small, 

 best when two or thrae inches high, and before the stem and flower 

 are formed with a 2 or 3 per cent solution of the salt, at the rate of 

 about 40 gallons per acre. To be successful, the operation should be 

 performed in dry, sunny, calm weather. It is then found that the 

 charlock leaves blacken and the plants die, while the barley and 

 clover not only are not injured, but appear, in many cases, to be 

 benefited by the process. This plan of dealing with charlock was ap- 

 parently first tried in 1897 by Girard in France. He used a 5 per 

 cent solution and ascribes the destruction 01 the charlock to the 

 poisonous effect of the. solution, which would be retained on the rough 

 and more or less horizontal leaves of the charlock, while it would 

 quickly run off the smooth and erect leaves of the cereals. It is. 

 doubtful whether this opinion is entitled to much weight, as clover, 

 which also has horizontal leaves, suffers little or no damage. It is 

 possible that the action is in some way dependent upon the presence 

 in the charlock (as in other crutiferez) of organic sulphides or sulpho- 

 cyanides, and that some reaction of the copper upon these compounds 

 is the cause of the injury. 



Another possible explanation of the toxic action of copper and 

 iron sulphate solutions is that, in contact with the cells of plants, 

 osmo.ic pressure is set up owing to the liquid outside being more 

 concentrated than that in the protoplasm of the cell. Water there- 

 fore leaves the protoplasm, and shrinkage occurs ("plasmolysis"), so 

 that the vital processes of the plant are interfered with, perhaps by 

 the destruction of the continuity of the protoplasm. If, through dif- 

 ferences in the strength and thickness of the cell walls, this action 

 takes place more readily in such plants as charlock, etc., than in 



1 Farmers' Bulletin, No. 75, U.S. Dept. of Agric. 



