829 



to the interior. It is therefore necessary to obtain a part of the in- 

 fected potato presenting it in a comparatively dry state, and this is 

 very easy, foi on breaking open some specimens, little accidental ca- 

 vities or fissures present the spawn in all its luxuriancy, and it is 

 precisely identical with that above referred to on the leaf, lining the 

 hollows which it has reached. In cavities which are excluded from 

 the air the fruit-bearing stems of the fungus are not thrown up from 

 the spawn, but in favourable spots they are as evident as upon the 

 leaves. In every instance the disease of the substance of the tuber is 

 most obviously in communication with its outer surface ; small brown 

 spots, stains, or lines, marking with certainty the route of the infec- 

 tion from without inwards. If a single instance can be adduced of 

 the origin or existence of the disease in the centre of the tuber, inde- 

 pendently of this communication with its surface, then this theory 

 and the results of my observations are entirely shaken. In every in- 

 stance, I have found it advance from the circumference to the centre. 



"I have described the stems and oval fruit-bearing heads of the fun- 

 gus. The cottony ground-work, or spawn, when highly magnified, 

 is found to be composed of jointed or vesicular filaments, infinitely 

 branched, and which, especially upon the cut surfaces of diseased 

 tubers, become heaped together, a white mass being thus formed, 

 bearing a resemblance to snow. 



" In cellular plants, as mushrooms, the rapidity of growth is most 

 surprising, and botanists, attempting the calculation, have supposed 

 that many millions of cells are formed in a single hour. I introduce 

 this analogy of the growth of other vegetable formations to reconcile 

 the seeming paradox of the plant under consideration advancing 

 with such frightful rapidity. If a diseased potato be cut in two, and 

 having been laid aside for a day or two, be then examined, the por- 

 tions of the cut surfaces stained by the infection, will be found 

 covered with a white mould, which is entirely composed of the web- 

 like filaments. 



" The inductions and general remarks derived from the investiga- 

 tion are as follow : — 



" 1. The blight first attacks the leaf of the plant, and originates 

 from invisible germs or prolific particles diffused (perhaps universally) 

 through the air, and which await a favourable opportunity for their 

 development, just as other parasites, whether Fungi or flower-bearing 

 plants, select their own proper soil — thus the mistleto selects the 

 apple or oak ; the Cuscuta, or dodder, the wild thyme and furze ; a 



Vol. it. 5 l 



