90 Mr. Chum on the Potato Disease. 



XIV. Artificial Production of the Potato Disease. 

 By Walter Crum, Esq., F.R.S. 



On grating down a healthy potato, the surface of the pulp, or the part 

 of it immediately in contact with the air, soon acquires a flesh-red colour, 

 which goes on increasing in depth to a mahogany brown. In a few hours 

 this is changed into a sooty black colour, such as occurs in certain stages of 

 the potato disease ; and at last, after five or six days we have again a brown 

 colour, similar to what appears in that stage of the disease when the part 

 has lost its firmness. This is a well-known process of putrefaction. It 

 occurs in the apple, where a part that has been bruised very soon becomes 

 brown. And the cause is also well understood to be the rupture of the 

 vessels or bags in which, while the fruit remains entire, the saccharine 

 matter is contained and kept apart from the nitrogenous or fermenting 

 principle. The grape also, in which the solution of sugar is contained in 

 cells distinct from the gluten, may be preserved for a long time unchanged; 

 but as soon as it is bruised, and the contents of the various cells are 

 thereby allowed to mix together, the gluten, by attracting oxygen from 

 the atmosphere, becomes converted into yeast, and fermentation goes on. 

 By the continued exposure of such mixtures to the air, putrefaction ensues, 

 and the conditions are fulfilled for the development of fungi. Such is the 

 case when the potato is broken up and exposed. Its sap, which contains 

 albumen (similar in composition and properties to the white of egg), and 

 occasionally also casein, is thus brought in contact with the other ingredi- 

 ents of the root and with the air. The consequence is a commencement 

 of putrefaction, and the production of a disease, to all appearance similar 

 to that which has occurred in nature during the present year. Examina- 

 tion by the microscope confirms their identity. In two or three days a 

 mouldiness appears upon the surface of the blackened pulp, consisting of 

 fungi with long stalks and globular heads, which emit when compressed 

 a profusion of small round bodies, called sporules, the seeds of new fungi. 

 These seeds are in no danger of being confounded with the granules of 

 starch, most of which in comparison with them, are several hundred times 

 as large. Lastly, after an exposure of eight days (and my observations 

 extend over no longer time), when the pulp has in a great measure lost 

 its blackness, and taken the (I believe more permanent) brown colour, 

 small, extremely white, and fine tufts appear on its surface, of a totally 

 different variety of fungus, having apparently no head like the earlier 

 crop, and consisting of long slender stems, which, when pressed down 

 between pieces of glass, appear lined on both sides with multitudes of 

 very small sporules. This fungus corresponds with the tufts which grow 

 on the outside of the diseased part of the potato. Their appearance is 

 the same, but any specimens of the tuft from the diseased potato I have 

 at present at command, are much older than the crop of which I speak, 

 and perhaps for that reason show fewer sporules. That a rupture of the 

 cellular tissue of the diseased potato has actually taken place during the 



