On Potatoes. 



rally inspissated state of the sap in the dry and farinaceous 

 varieties. I conceived that the sap, if not sufficiently fluid, 

 might stagnate in, and close, the fine vessels of the leaf 

 during its growth and extension, and thus occasion the irre- 

 gular contractions which constitute this disease ; and this 

 conclusion, which I drew many years ago, is perfectly con- 

 sistent with the opinions I have subsequently entertained, 

 respecting the formation of leaves. I therefore suffered a 

 quantity of Potatoes, the produce almost wholly of diseased 

 plants, to remain in the heap, where they had been preserved 

 during winter, till each tuber had emitted shoots of three or 

 four inches long. These were then carefully detached, with 

 their fibrous roots, from the tubers, and were committed to 

 the soil; where having little to subsist upon, except water, 

 I concluded the cause of the disease, if it were the too great 

 thickness of the sap, would be effectually removed ; and I 

 had the satisfaction to observe, that not a single curled leaf 

 was produced ; though more than nine tenths of the plants, 

 which the same identical tubers subsequently produced, 

 were much diseased. 



In the spring of 1808, Sir John Sinclair informed me 

 that a gardener in Scotland, Mr. Crozer, had discovered 

 a method of preventing the curl, by taking up the tubers 

 before they are nearly full grown, and consequently before 

 they became farinaceous. Mr. Crozer, therefore, and my- 

 self, appear to have arrived at the same point by very dif- 

 ferent routes ; for by taking his Potatoes, whilst immature, 

 from the parent stems, he probably retained the sap nearly 

 in the state to which my mode of culture reduced it. I 



