INTRODUCTION. 5 



the attack of its enemies, succumbs. The number of plant parasites is 

 immense, and they are found in all classes of animated beings (phane- 

 rogams, cryptogams, bacteria) as well as animals (worms, insects, acari) 

 living at the expense of the plant. They all become more dangerous, the 

 one more than the other, in so far as they find the medium favourable 

 to their development and extraordinary multiphcation — a medium of 

 constant and invariable composition, and climatic conditions lying be- 

 tween narrov7 limits, to which are intimately linked the growth and 

 reproduction of each organism. Between enemies the struggle is con- 

 stant. The reaction of the plant against the parasites which threaten 

 it, its cellular activity, which opposes to them its layers of bark which 

 creates deposits of tannin, of acids in the cells, layers of wax on the 

 epidermis, prevent it from succumbing. There is no disease until the 

 reactive forces of the plant become powerless to prevent the develop- 

 ment of parasites, until the disposition of the subject, and special and 

 exceptional conditions, facilitate their evolution, increasing their viru- 

 lence and their number. There is, however, disease when the parasitic 

 antagonists imported from a foreign country (as was the case with 

 certain insects imported from America, and as is seen in America as 

 regards insects of European origin) are deprived of their natural 

 parasites capable of hindering their abnormal multiplication. Great 

 invasions of parasites must be regarded, in fact, as accidents, for nature 

 has attached to each ravager one or more parasites which live at its 

 expense, just as it itself lives at the expense of the plant. These 

 parasites obey the same laws as the ravagers, multiply with the same 

 rapidity as the latter, and by diminishing their number is the check 

 which nature opposes to the abnormal multiplication of the species. 

 Moreover, sudden changes of temperature at the time of the casting of 

 the skin of the larvae of the insects — then very delicate — frosts, heavy 

 rain, free electricity', appear to be some of the causes which hinder the 

 development of too great a number of parasites. In nature these 

 causes prevent epidemics, which would be very rare if man did not, 

 by his methods of culture, create specially favourable conditions for the 

 evolution and multiplication of parasites. Formerly, this state of affairs 

 was remedied by the bare fallow ; to-day, rotations are preferred ; to- 

 morrow, the annual disinfection of the soil by means of carbon disul- 

 phide, that of the underground and aerial (above ground) part of plants 

 by insecticides and anticryptogams will definitely abolish a condition 

 which necessarily results from our methods of cropping. We must re- 

 place by new processes the action of stable equilibrium which is mani- 

 fested in nature and which we have suppressed. It will be seen, however, 

 that the complete destruction of parasites is not indispensable, but even 

 injurious, and that disinfection should only re-establish equilibrium, 

 a modus vivendi between the plant and its parasites. It is necessary, 

 in fact, to avoid diminishing in the plant the reactive force of the cells, 

 so that these may always be armed 'and active, and able at any moment 

 to sustain the struggle against parasites. This equilibrating disinfection 

 is only reahzed, up to now, in the treatment of the vine against 

 Phylloxera and cryptogamic parasites by the annual treatment (a) of 

 the soil by weak applications of carbon disulphide, of dissolved sulpho- 



