ME. BEKKELEY ON THE POTATO MURRAIN. 15 



of the leaves appeared white mealy patches, consisting of a 

 minute mould proceeding, either singly or in fascicles, from the 

 stomata, and arising from an abundant branched mycelium creep- 

 ing in every direction through the loose tissue beneath the cuticle. 

 The upper surface rarely if ever exhibits the mould, it being 

 almost physicall)^ impossible for its delicate threads to penetrate 

 the closely packed cells which, being arranged side by side, leave 

 scarcely any intercellular passages. The mould in a few hours 

 from its first piercing the apertures of the stomata perfects its 

 fruit, and in so doing completely exhausts its matrix, which in 

 consequence withers. No sooner have a number of the leaves 

 been attacked than the stem itself is subject to change, becoming 

 spotted here and there with dark brown patches in which the 

 cells are mostly filled with a dark grumous mass, without exhi- 

 biting any mucedinous filaments, though occasionally, as in a por- 

 tion figured, I have ascertained their presence. Very rarely 

 fructifying but dwarfed specimens of the mould occur upon it. 

 I have figured such, as sent to me from the neighbourhood of 

 Bristol by Mr. Broome. The stem now rapidly putrifies, the 

 cuticle and its subjacent tissue becoming pulpy, and separating 

 when touched from the woody parts beneath. The whole soon 

 dries up, and, in many instances, exhibits in the centre the black 

 irregular fungoid masses which are known under the name of 

 Sclerotium varium,* aiid which are believed by the most compe- 

 tent authorities to be the mycelium of certain moulds in a high 

 state of condensation. 



If the tubers are now examined, the greater part will often be 

 found smaller than usual, especially if the disease has commenced 

 at an early stage of growth, but in their natural condition, while 

 here and there a tuber, particularly if it has chanced to be par- 

 tially exposed, exhibits traces of disease. The surface is, however, 

 soon marked with livid patches, commencing generally about the 

 eyes, or at the point of connection with the fructifying shoots, 

 according to Payen and Phillips ; these rapidly acquire a spotted 

 appearance, the spots being rather waved, and assuming often a 

 more or less concentric arrangement. Sometimes, especially on 

 the smoother kinds of tuber, two or more regular systems of 

 concentric spots are exhibited on the same tuber. The skin now 

 withers, and is easily separated; the spots become depressed, 

 and of a yellowish tinge ; and if the tubers be laid in a moist 

 place, in a day or two — sometimes in the space of a few hours — 



* This has been the case very generally. I have received specimens from 

 Mr. J. D. C. Sowerby, procured near London ; from Suffolk by Professor 

 Henslow ; from Dumfrieshire by Sir W. Jardine ; and from Elgin by Mr. 

 Robert Arthur, who communicated them to the London Horticultural Society. 



