REMEDIAL MEASURES 45 



5. The Sulphates of Iron and Copper have been very 

 extensively employed on arable land for the destruction 

 of weeds among corn and other crops, and these sub- 

 stances may be regarded as the only materials at 

 present of practical importance for such purposes. 



To be effective they must be applied in solution 

 and absorbed by the tissues of the plants, and as such 

 solutions have only a local action and are not dis- 

 tributed through the body of the plant as materials 

 absorbed by roots would be, the whole or a great part 

 of the leaf-surface of the plant must be wetted if the 

 weeds are to be killed. Moreover, since the only parts 

 which can be got at are the leaves and stems above 

 ground, spraying is most effective upon weeds of annual 

 duration which have no reproductive machinery upon 

 their roots. Perennials which maintain a store of food 

 in tubers, and thickened rhizomes and similar struc- 

 tures in the soil, cannot be readily destroyed by 

 applications of poisons to their leaves, for after the 

 latter have been destroyed fresh stems and leaves may 

 be sent up from buds on the underground parts. 



Even when solutions are applied to the sub-aerial 

 leaves and stems, the result depends not only upon the 

 nature of the chemical substances and the strength of 

 the solution used, but also upon the kind of plant, its 

 habit of growth, the stage of development which it has 

 reached, and other factors. As a rule, the young grow- 

 ing points of the stems of plants are enclosed in the 

 interior of buds, or are protected, as in the grasses, by 

 the surrounding leaf-sheaths. While the older exposed 

 leaves may be killed, the weed may still be able to grow 

 from its protected buds and to perfect its seeds. 



Plants with small, narrow, upright leaves are not so 

 easily destroyed as those with broad, spreading leaves, 

 for the solution runs off the former more readily than 



