CH. VIII.] THE DISEASES OF PLANTS. 265 



sovm with seed from the same growth, will one be 

 diseased and the other healthy. 



Secondly, It does not arise from an unfavourable 

 time of sowing, or from dry unpropitious seasons dur- 

 ing their aftergi'owth ; " for, on this supposition, we 

 might expect that in all turnip districts the disease 

 would occasionally make its appearance, in conse- 

 quence of variations in the period and mode of sow- 

 ing, or from following droughts ; yet we know that 

 in many parts of the country it has never been 

 heard of." 



Thirdly, It does not arise from the quality of the 

 soil ; for Sir Joseph Banks suffered from its infecting 

 thin stapled sandy fields, whilst all Holdemess, 

 which is generally a strong loamy soil, was found 

 equally liable to the disease. But a stiU more 

 decisive e\ddence on this point is, that it makes its 

 appeamnce at uncertain intervals upon the same 

 soil ; the turnips upon it being in some years greatly 

 injured by the disease, and in other years entirely 

 free. 



Fourthly, Although it is certain from the observa- 

 tions of Sir Joseph Banks, and from general experience, 

 that the disease occurs most frequently in soils tired 

 of the crop, that is soils upon which it has been grown 

 for a long course of years ; yet, that this is not the 

 immediate cause of the disease is proved by the 

 fact, that often only patches in the same field are 



