'20 INSKC'TK'IDKS, FUNCtK'I]-)ES, AND WEKD KILLERS. 



two cases are easily explained, the lir?t by the fact that the bacteria 

 seek an alkaline medium created by the lime, the second by the fact 

 that the Sclerotina seeks, on the contrary, an acid naedium created by 

 the acid phosphate. Intense nitrogenous manuring favours the de- 

 velopment of phytophora. It is thus essential to avoid the use of 

 manures which may place the plant in a state of subjection in the 

 struggle whicli it has to sustain against inimical factors. 



Choice of Species. — One species may be more subject to disease 

 than another, and possess a predisposition for certain pathological con- 

 ditions. That occurs when the conditions favourable to the develop- 

 ment of the plant are also those which favour the evolution of parasites 

 at the time when the plant is young and possesses delicate tissues 

 just when the parasites are most virulent. Care must be taken in 

 sowing a plant that the germination of the seed does not coincide 

 with the virulent development of the parasite, with the ripening of the 

 spores of fungoid enemies, or the hatching of the eggs of certain 

 insects. It suffices to sow a little earlier or later. But in spite 

 of all that can be done to eliminate parasites, they none the less exist 

 and ravage the tissues. The cells of the plant, like those of the human 

 organism, react, and it is found that after this constant struggle they 

 undergo certain modifications which are opposed to the develop- 

 ment of the parasite, and the plant acquires a certain immunity. It is 

 acknowledged that the deposits of tannin and other materials in 

 certain cells and the concentration of the sap are conditions resulting 

 from the struggle of the plant against insects, and destined to oppose 

 an unsuitable medium to any attempt of development. Plant 

 diseases do not, therefore, depend solely on the presence of a parasite, 

 but as much on the conditions predisposing the plant to a want of 

 reactive energy, and it has been found that this predisposition was an 

 attribute of certain species or certain varieties. 



Meteorological Influences. — Although we are still badly equipped 

 to struggle against atmospheric influences, each day brings new dis- 

 coveries from which agriculture knows how to benefit. Thus hail 

 and morning frosts may be effectively prevented — hail by artificial per- 

 cussion of the atmospheric layer where hail is formed, morning frosts 

 by means of artificial clouds. Without neglecting therapeutic 

 methods it is necessary to take incessant prophylactic measures to 

 prevent the evolution of diseases and their propagation, to treat the 

 seed, the plant, the soil, and the crops by toxic products, to destroy 

 the plants invaded, which form hot-beds of infection, to avoid the im- 

 portation of plants from districts notoriously infected. Effort must 

 be made to apply a general ti-eatment to the plant, to remove as far 

 as possible all conditions favourable to the growth of parasites. The 

 hygiene of the plant must receive careful attention ; sowing retarded 

 or advanced ; the plants protected against eventual frosts and hail ; 

 drain and lime the soil against humidity, the great predisposing cause 

 of cryptogamic diseases ; apply appropriate strengthening manures ; 

 choose hardy species obtained by crossing or by selection, and create 

 new varieties combining great resistance to plant diseases with the 

 necessary properties of production. So that the struggle may be sue- 



