12 10 MEANS OF PREVENTION AND CONTROL 



stances, to increasing the resistance of plant organs against their parasites. 

 For this reason, the varieties or races of fruits or grapes which are intense in 

 colour, from red to violet and bluish, are always more resistant in compari- 

 son with varieties of the same species with more or less pale fruits. 



4) The tannins co-operate with the other organic acids in defending the 

 organs of the plants even against their animal parasites ; hence the pure or 

 hybrid American races of vines, richer in tannic acid, are always more re- 

 sistant to phylloxera, other conditions being equal. 



Selection, Crossing and High Grafting. — 1) The resistance to adverse 

 factors varies in the same environment, just as to the variety and the races 

 of one and the same cultivated species. This resistance being inherited, it 

 is essential to maintain it by renewed selection, the value of which is almost 

 exclusively local. 



2) Resistance is greater in the varieties usually held in least esteem ; 

 in this case, when selection seems insufficient, it is indispensable to resort 

 to hybridisation, especially' for grasses, and to a high grafting on hardy stocks 

 for fruit plants, unless reproduction by seed is resorted to. 



3) With similar processes there have also been produced resistant 

 races : {a) of wheat to rust, smut and lodging ; {b) of buckwheat, sugar cane 

 and stone fruit Rosaceae, to heavy frosts ; (c) of asparagus to rust ; {d) of 

 tomatoes, cotton tree and water melons, to wilt ; {e) of potato, to mildew ; 

 (/) of strawberries, to mould; [g) of vines, to oidium, mildew and phylloxera; 

 (h) of citrus plants, to gummosis and root rot. 



Manuring. — i) Potassium, lime, iron and magnesium must be appUed 

 to the plants in the form of sulphate exclusively ; the separation of the cor- 

 responding ions, which is done by the roots, would set at libert)^ the sulphuric 

 acid, which, even in a very small quantity, would, by recombination, 

 increase the acidity of the juice circulating in the organs of the plants and 

 render resistance stronger. 



2) Nitrate of soda is the nitrogenous manure which, more than any 

 other, tends to reduce the acidity of the organs and render them more sensi- 

 tive to enemy action ; sulphate of ammonia, on the contrary, causes a produc- 

 tion which is somewhat less in quantity but renders the plants more resistant, 

 because it tends to increase the acidity of their organs; in alkaline soils, the 

 use of nitrate of soda will, by accumulating sodium, result in diminishing 

 the original fertility and will produce progressive sterility. 



3) An excess of nitrogenous manure, especialh' in the nitric form, 

 causes an increase in the formation of starchy products which, like sugars, 

 render the organs more susceptible to parasites. On the contrary, phosphatic 

 manure produces a transformation of the soluble nitrogenous substances 

 (or starchy substances) into insoluble nitrogenous substances (or phospho- 

 proteins) which possess a negative chemotropism. This manure, while it ac- 

 celerates the ripening of the fruit and lignification of the branches tends to 

 protect them from the attacks of external agents and at the same time con- 

 tributes to maintaining or increasing the acidity in the organs, thus streng- 

 thening them in their struggle against parasites. 



