24 ME. BERKELEY ON THE POTATO MURRAIN. 



matter, but to produce decay— a fact which is of the first import- 

 ance. Though so many other species have this habit, these have 

 not. The. plant then becomes unhealthy in consequence of the 

 presence of the mould, which feeds upon its juices and prevents 

 the elaboration of nutritive sap in the leaves, while it obstructs 

 the admission of air and the emission of perspiration. The stem 

 is thus overcharged with moisture and eventually rots, while 

 every source of nutriment is cut off from the half-ripe tubers. 

 It would be as reasonable to say, with our knowledge of the 

 nature and habits of the cereal fungi, that bunt, or mildew, or 

 the other allied diseases which affect corn, are the consequence 

 and not the causes of disease. In favourable seasons they are 

 not developed ; in unfavourable seasons they spread like wildfire : 

 in one sense, therefore, the atmospheric conditions are the cause, 

 but merely as they stimulate into action the latent pest. The 

 immediate cause of disease is the fungus which preys upon the 

 tissues of the corn. So exactly, in the present instance, as far 

 at least as the aerial portions of the plant are concerned, the Bo- 

 trytis is the immediate cause of destruction. In some instances 

 it may have been aided by unseasonable frost, but this has cer- 

 tainly not always been the case. The mould indeed would not 

 have spreafl but from peculiar atmospheric conditions favourable 

 to its growth. What these are it may be impossible to say, but 

 it is a fact well known to every student of the extensive tribe of 

 Fungi, that their growth, and especially their numbers, depend 

 more than all other vegetables on atmospherical conditions, or 

 what Fries has happily called " cosmica momenta." Even the 

 peasant knows this to be the case with mushrooms. Dry and 

 wet summers occur, and both are equally barren ; while in other 

 seasons, apparently but little dissimilar, they occur in the utmost 

 profusion. A species will be most abundant for a year or two, 

 and then for a period vanish entirely. It is notorious that this 

 is the case in other parts of the creation, especially amongst 

 insects, peculiar species of which sometimes swarm to such an 

 extent as to baffle the naturalist. In the summer of 1826, for 

 instance, Vanessa cardui existed in the greatest profusion in Eng- 

 land, and it was traced by Mr. A. Way from England to Nice. 

 The species of late years has been comparatively rare. There is 

 nothing surprising then in the fact of the immense prevalence of 

 a parasitic mould. No one wonders when the hop-grounds are 

 ravaged by their peculiar mildew, because the cultivation of hops 

 is so limited ; but if it were as universal and of as much import- 

 ance as potatoes, the ravages would equally excite attention. It 

 is by these instruments, contemptible in the sight of man, that the 

 Almighty is pleased sometimes to accomplish his ends. In- 

 stances, like that of the Hessian fly, will readily occur of the 

 immense disproportion between the means and the end. 



