INTRODUCTION. 9 



conditions the sequence of their uninterrupted evolution. The chemical 

 agents do not therefore necessarily kill the parasites and their organs 

 of propagation ; often they only paralyse for a certain time the 

 normal evolution of the parasite. The more the therapeutic agent is 

 capable of insolubilizing albumen, or of modifying the substances 

 constituting the cells, the more active it is. Wuthrick (" Zeitschrift fiir 

 Planzen krankeiten de Sorauer," 1892, p. 81) examined the comparative 

 action of various substances on the different spores of fungi. His 

 researches, in which mention is made of the relation which exists 

 between the molecular weight of the chemical products and their 

 action on parasites, leave no room for doubt on the subject of the 

 sinirilarity of the action of various chemical products on the vital 

 substances of parasites. Other poisonous chemical products act on 

 parasites, some in virtue of their properties as solvents of organic 

 matter, such as the caustic alkalies, alkaline soaps in aqueous or 

 alcoholic solution, and certain acids, others by their dehydrating 

 action exerted chiefly on the medium on which the parasite lives. 

 No parasite living in an aqueous medium can develop except when 

 the amount of water contained therein does not descend below a 

 minimum. A disease may be stopped if the conditions of existence 

 of a parasite be modified in this direction. Other chemical products 

 are asphyxiants : impalpable powders, and oils and fats ; they obstruct 

 the respiratory passages. 



Action of Chemical Products on Plants. — The chemical pro- 

 ducts used to combat plant diseases have all, to a certain extent 

 unless insoluble, an injurious action on plants. The plant is gener- 

 ally less sensitive to chemical agents than the spores of fungi, and 

 more sensitive than insects, their larvae, and their eggs. 



Liquids Spread on the Surface of Plants may penetrate therein 

 by endosmosis, whilst gases and vapours do not appear to be, or are 

 with difficulty, absorbed by the plant. 



It follows that the treatment of plant diseases may be preferably 

 done — 



1. By iiroducts under the form of gas or vapour. 



2. At a time ivhen the organs lohich ])ermit endosmosis no longer 

 exist, and luhen the cellular activity of the plant is reduced to a mini- 

 mum, that is to say, in lointer. 



At that time of the year chemical agents of any degree of concen- 

 tration may be used without injuring the plant, whilst in summer 

 infinite precautions must be taken not to destroy the organs of the 

 infected plant at the same time as the parasites. Treatment by gas 13 

 very efficient, and is becoming more common every day, whether it 

 be the treatment of the part of the plant above ground (aerial), or of 

 that underground, the roots being infected as frequently as the stems 

 by parasites injurious to their normal evolution. With this end in view 

 injections of carbon disulphide, petroleum, benzene are made into the 

 soil, and by enclosing (clochage) the part above ground, an atmosphere 

 may be created charged with sulphurous acid, carbon disulphide, 

 prussic acid, nicotine, etc. When solutions or emulsions of the 

 chemical agents are to be used in spring and in summer, the sensitive- 



