I206 MEANS OF PREVENTION AND CONTROI, 



tioii methods, and nitrogenous manure are dealt with in particular. The 

 conclusions are as follows : 



Climate. — i) A plant cultivated in a climate differing from that of its 

 origin, retains and may even increase its resistance to adverse agencies 

 if introduced into a colder climate ; on the contrary, it loses this resistance 

 in an ever greater degree if introduced into a hotter climate. 



2) The necessity of adapting itself to the ne\y climate, even if the 

 difference from that of origin is small, produces variation in the physio- 

 logical processes of the imported plant ; it inevitably entails a greater v^aria- 

 tion in the quantity than in the quality of the organic substances it produces, 

 and consequently of those also which furnish the plant with the necessary 

 means for resisting the attacks of its enemies. 



3) The variations in the cell sap produced by a colder or a hotter 

 environment are reproduced in analogous fashion in plants cultivated with 

 a colder (northern) exposure or hotter (southern) exposure respectiveh\ 



4) These variations form a local problem as regards the selection of 

 the most resistant varieties. 



5) The plants imported into hotter and moister, and into wetter lo- 

 calities always become more sentitive to disease, and in particular to the 

 attacks of parasitic micromycetes. The same is the case in exclusively 

 rainy years during the growth of the plant. 



6) Repeated and conclusive examples exist of the fact that plants 

 imported into hotter climates than those of their origin gradually lose their 

 original resistance to adverse factors, as in the case of wheat with regard to 

 rusts, the vine with regard to oidium, mildew and phylloxera, and generally, 

 kitchen garden and fruit plants in relation to their respective parasites. 



7) Comparisons between the behaviour of plants in different cli- 

 mates can only have a conclusive value in those cases where all the con- 

 ditions of cultivation of the several plants are alike; otherwise, the conclu- 

 sions will be false or at any rate arbitrary. 



Soil. — i) The soil water having even a slightly acid reaction more 

 effectively facilitates the growth of the plants cultivated therein. 



2) Soils with an alkaline reaction, on the contrar3^ facilitate the phe- 

 nomena of drying up of the leaves in oats, of scurf in the potato, or " foot- 

 rot " in wheat, chlorosis in American vines, and so on. 



3) Limestone soils cause a greater production of sugar in plants, 

 and at the same time are an obstacle to the presence of free organic acids. 



4) The addition of alkaline substances to the soil renders plants more 

 sensitive to injurious external influences; acid fertilisers and manures have 

 a contrary effect. 



5) The resistance of plants to adverse factors is greater in loose soils 

 and less in compact soils. 



6) Stagnant siibsoil waters, owing to their gradual impoverishment 

 in dissolved oxygen, cause asph^'xia of the roots, which is followed by ne- 

 crosis of the tissues, and afterwards by the appearance of mycelia (rhizo- 

 morphs, Rhizoctonia, etc.). 



7) The hardiness of wild plants is due not only to the greater den- 



