827 



" The potato plant, it is almost needless to observe, contains a 

 quantity of water, which greatly exceeds that of all its other ingre- 

 dients, the proportion contained in the tuber being nearly 80 per 

 cent. — starch forming the principal of its remaining elements ; the 

 vegetable gluten, saline matters, &c., existing in very minute propor- 

 tions. It is therefore a weakly plant and, perhaps, on that account, 

 more easily the prey of certain minute vegetable germs which the at- 

 mosphere may be supposed always to contain in great abundance, 

 and which (each species selecting its own soil) only await congenial 

 occasions to render themselves visible in the growth they originate, 

 whether on old and damp books, rotten sticks, plants, &c. 



" Fungi of this class generally attack decayed substances, or such 

 as possess weak power of life : the pea plant, for instance, whose de- 

 cay is rapid as its growth, is peculiarly liable to one species, which, 

 magnified, is a most beautiful object: its spawn, or web-like fila- 

 ments spreading upon the surface of the leaf, and here and there 

 throwing up fruit-bearing heads — a little field of microscopic mush- 

 rooms. Indeed, its mode of coursing the surface of the pea-leaf is 

 precisely similar to the growth of the potato fungus. 



" I think it necessary to refer briefly to the healthy potato plant, as 

 some of its appendices are of remarkable formation : I mean chiefly 

 the small hairs or bristles which are scattered on both sides of the 

 leaf, and more thinly on the stem, and also, small globular bodies 

 supported on footstalks, which possibly may be the rudiments of 

 the future hairs, which are jointed, and liable to modification with 

 the age of the plant. In the mature or decaying plant, these joints 

 become nearly perfect cones, the base of each being very slenderly 

 attached to the apex of the cone beneath, presenting a very singular 

 appearance These peculiar hairs, or perhaps glands, must not be 

 mistaken for the parasitical fungus, from which, indeed, they greatly 

 differ. 



" The tuber of the potato is cellular, the cells being pentagonal, 

 each containing several starch grains (fi:om 8 to 20), which are oval, 

 and perfectly unconnected with each other. If a tuber be cut and 

 exposed to the sun and air, the cells will, in the process of drying, 

 contract, and force to the surface a layer of starch grains, which have 

 a whitish, crystalline appearance. 



" 1. TJie Disease of the Leaf. 

 " The appearances to be described must not be confounded with 

 occasional brown spots and holes, which are obviously the work of 



