28 MR. BERKELEY OX THE POTATO MUKEAIN. 



The fact that the earliest potatoes, which had cold and wet 

 weather enough to contend against, were not attacked, is much 

 in favour of the fungal theory, the foliage never having suffered. 

 The second crops suffered perhaps nearly as much as the later — 

 in some cases more so. 



The circumstance of the concentric arrangement of the spots 

 in certain cases appears to me of great importance, as indicating 

 their origin from a fungus. I regret much having searched in 

 vain for fresh specimens of Oidium fructigenum, the species on 

 which Ehrenberg made his observations on the growth of fungi, 

 with a view to ascertain the condition of the mycelium. As the 

 texture of the matrix, however, is so loose, it would be probable 

 that it would be freely developed, as is the case with that which 

 is so abundant in the ordinary decayed spots of pears and apples. 

 The reports are so contradictory as regards the subject of con- 

 tagion, that it is impossible to build anything upon them.* Some 

 deny the fact altogether, while others say that a single diseased 

 tuber is sufficient to infect a thousand. When it is considered 

 that apparently sound tubers will, even tliough exposed to light 

 and kept dry, in a few days exhibit the disease, and that in a 

 moist place the change takes place more certainly, be the selection 

 as careful as it may, it appears clear that no experiment of 

 sprinkling them with the water in which diseased individuals 

 have been boiled, or confining them within the same atmosphere, 

 or even of placing apparently sound individuals in contact with 

 unsound, can be conclusive. I have seen no evidences of con- 

 tagion in the experiments I have myself instituted. 



Were a single fungus only produced upon the tubers, viz., 

 that which grows upon the leaves, there would be less difficulty, 

 I believe, in admitting tiie agency of the Botrytis. The difficulty 

 is, I think, however, overrated : when it is considered how exten- 

 sively — nay, universally — moulds are produced in decaying mat- 

 ter, and how many species occur upon one putrescent mass, it will 

 not seem so wonderful that when the Botrytis has superinduced 

 decay, other species should follow in its wake. If the peculiar 

 spots, in the first instance, were found on extensive trial to pro- 

 duce indifferently a variety of species, the case would be different ; 

 but if, as I believe, the Botrytis always precedes when the trial 

 is sufficiently early, the conclusion is evident. 



Before closing- this part of my subject it may be well to men- 

 tion an observation which I made during the course of my in- 

 vestigation. On examining the cnticular cells of a young tuber, 

 •with a view to ascertain the changes which occur in the process 



Payen thiuks it not contagious : Morren, on the contrary, contagious. 



