14 MR. BERKELEY ON THE POTATO MURRAIN. 



microscopic insects, to an epidemic resembling cholera, to the 

 practice of raising potatoes constantly by division of the tubers, 

 and especially from tubers cut in spring, to the use of animal 

 manure, or to the degeneracy of the plant itself, but without any 

 data upon which to found their assertions : others, again, to the 

 influence of a parasitic fungus which first attacks the leaves and 

 ultimately the tubers. Before reviewing these opinions it will 

 be well to describe the phases exhibited by the disease, reserving 

 to the end of the memoir the more purely botanical points con- 

 nected with the subject. The chemical part of the question does 

 not fall within my scope, and the remedial measures have been 

 already exhibited in a form generally accessible to English 

 readers in the reports of the Irish Commissioners. 



The progress of the disease has been described in almost the 

 same terms by all who have written on the subject. It was at 

 first indeed believed by many that it began in the tubers and was 

 propagated upwards, but every one seems pretty well convinced 

 at present that the order of events is precisely contrary ; for 

 though it is true that in diseased fields a few scattered tubers, 

 principally if not entirely such as chance to be superficial, may 

 occasionally be found, or even a diseased tuber, on a plant whose 

 leaves and stalks are apparently healthy,* yet these are quite 

 exceptional cases, the general fact being that, at the time the 

 foliage first becomes diseased, the tubers are healthy. 



The potato crops up to a certain time were very luxuriant, 

 as testified indeed by the produce in spite of the premature decay 

 of the organs on which of course the perfection of the crop de- 

 pends. This was especially the case with the crops which were 

 attacked in Canada in 1844, and Dr. Bellingham informs us that 

 in Ireland there was " a luxuriance of the leaves, flowers, and 

 stalks, which led most people to suppose that there would be an 

 unusual yield." The luxuriance perhaps indicated the latent 

 disease in accordance with the phenomena presented by corn 

 affiscted by cereal fungi.t The leaves then began suddenly to 

 assume a paler and at length a yellowish tint, exhibiting here 

 and there discoloured spots, and, if I mistake not, were less copi- 

 ously clothed than usual with pubescence on the under surface. 

 More or less exactly coinciding with these spots on the reverse 



* Eevue Botanique, 1845, p. 150. 



t It is well known that the presence of the mycelium of fungi acts as a sti- 

 mulant to the chlorophyl : witness the rich tint of fairy rings. A curi- 

 ous instance has within a few days fallen under my notice. The hazel leaves 

 a week or two back were very generally spotted with dark patches of green. 

 On examination it was found that the reverse of such patches was covered with 

 Erysiphe guttata, which had been living at the expense of the paler portions 

 of the leaf, while in the superjacent patch the chlorophyl had become of a 

 deeper green. 



