DISEASES. 173 



of the ftmgi that are believed to be the cause of the 

 trouble.* 



Poisons in the soil are frequently very deleterious to 

 vegetation, and we' often find extensive injuries to our 

 plants produced by this class of agents. When these are 

 of a chejaical nature, as is. usually the case, they may be 

 satisfactorily treated by applications that will neutralize 

 their effects. In cities the escape of the illuminating gas, 

 that is carried in subterranean pipes, has often so poisoned 

 the soil as to destroy the shade trees by the side of the 

 streets. 



An excess of certain saline and alkaline ingredients 

 often produces barrenness in the soil, by a sort of poison- 

 ing, even with those articles that in smaller quantities are 

 used as manures with the happiest effects. 



3d The influence exerted upon vegetation by the 

 growth of parasitic plants, cannot be observed without 

 forcing us to the conclusion that they are prejudicial to 

 the health of the plants they infest since they either cover 

 and smother the foliage by twining upon it, as is the case 

 with the Dodder; or fasten themselves upon a limb, ap- 

 propriating the sap that was intended for its support, and 

 thus starve it, as does the Mistletoe ; or attaching them- 

 selves to the bark, they interfere with its functions, as is 

 done by the lichens and mosses ; or,' following the descend- 

 ing scale, in the size of these parasites, but meeting in 

 them foes of much greater importance, we find the minute 

 but innumerable fungi attacking the wood, the bark, the 

 foliage, and the fruits, of our gardens and orchards, and 

 committing incalculable damage thus entailing serious 



* Cincinnati Hort. Soc. Report. 



