18 INSECTICIDES, FUNGICIDES, AND WEED KILLERS 



them. It is necessary to remove bad influences from plants, and 

 to supply them, if need be, in a regular and abundant manner with 

 the nutritive elements which they require. If it be asserted that a 

 disease can be transmitted to a plant by artificial infection when 

 placed in a laboratory where it has not all its means of reaction, it 

 must not be concluded therefrom that this same plant will always 

 succumb to this parasite in surroundings favourable to its develop- 

 ment and in good hygienic conditions. Owing to a special immunity 

 which is not acquired, except under certain conditions, the plant, on 

 the contrary, will be able to resist the attempts of invasion by 

 the parasites and will issue victorious from any struggle in all 

 instances. 



Most cryptogamic parasites are incapable of attacking the living 

 vigorous and healthy cell. Certain insects, even xylophagic, such as 

 the Scolytes, only attack a sickly tree, the intense motion of the sap 

 being injurious to the development of their larvae. On the other 

 hand, most parasites find an easy shelter in the plant when the latter 

 is enfeebled by an adjuvant cause, or when organs capable of being 

 invaded have been laid bare by a wound. 



Stimulants of Growth. — We know from the researches of 

 Raulin, Nageli, Pfeiffer, Richard, and Ono the favourable influence 

 which certain metallic salts absorbed by the sap can exercise on the 

 health of plants. Salts of iron, copper, mercury, zinc, nickel, cobalt, 

 manganese, lithium, fluorides, and arsenites have in a certain dose 

 a stimulating action on the vital functions of the plant, analogous to 

 that which arsenious acid exercises on our own organism. The use of 

 these stimulants may often be a useful means of stimulating the 

 vigour of the plant, and of rendering it more capable of resisting 

 ■cryptogamic diseases. 



Nutrition. — The researches of Liebig, Boussingault, Deherain, and 

 others have shown that the development of plants depends greatly on 

 the mineral elements which they find in the soil, and nothing is 

 more easy than to supply them when the soil is deficient therein. 

 The result of these researches has been intensive farming, which by 

 supplying in great abundance the elements, necessary for the growth 

 of plants has rendered it possible to double and triple the yield of 

 crops. Encouraged by such success we have learned to prepare an 

 exact account of the elements indispensable for each plant crop by the 

 analysis of its ash, of the elements of the soil, and taking into account 

 the nutritive elements that the preceding crop has removed and add- 

 ing to the soil the elements in which it is deficient. It has been ob- 

 served, however, that the plants obtained as a result of intensive manur- 

 ing were more subject to diseases, and that such assumed a dangerous 

 character. The great delicacy of the plants constituted a more favour- 

 able medium for their evolution, however little the climatic conditions 

 favour their development, and predispose the plants to infection. It 

 must be admitted that the intensive culture now practised does not 

 produce a normal condition of the plant, but a cultivated condition, 

 and that the parasites have acquired a greater vigour and become 

 more virulent owing to the great richness of the plant in nutritive 



