534 



leaves ; consequently, if the mould were the cause, the spawn pro- 

 ducuig it must have affected the plant at a very early stage of its 

 growth, and proceeded from the root in place of the stem. 



The Rev. Mr. Berkeley has accompanied his interesting memoir on 

 the potato-murrain with a beautiful series of microscopical drawings, 

 showing how this mucedinous plant grows, extending its filamentous 

 spawn under the cuticle of the leaf, and shooting up singly, or in fas- 

 cicles, through the apertures of the stomates, thereby obstructing the 

 admission of air, and the emission of perspiration, besides preventing 

 the plants from elaborating nutritive sap in the leaves. 



Although I still consider the mould only an accompaniment of the 

 disease, I am well aware of the vast devastation such minute plants 

 are capable of producing on the tissue of the larger vegetables. The 

 ravages of mildew, smut and bunt, are well known, and afford familiar 

 instances. The dry-rot — which was formerly such a dreadful pest in 

 rotting the timbers of ships and buildings, caused by the growth of 

 Merulias lachrymans — is another, but happily it has been, to some 

 extent, subdued since the process of steeping timber in certain saline 

 and acid mixtures became general. 



These parasites, further, are particular in selecting the victims for 

 their attacks, and confine themselves, in most instances, either to a 

 single species, or to those of a genus ; but occasionally, the same 

 species of parasite is found to grow on many of the plants belonging 

 to one natural family, and the conditions necessary for their develop- 

 ment are so peculiar as to preclude the possibility of effectually guard- 

 ing against them. The spores or seeds are so numerous, that if those 

 produced from a single head of diseased grain were all to vegetate, 

 they would be sufficient to infect a whole district; and I believe there 

 are few fields of wheat ever found effectually free from them ; but as 

 these minute spores depend so entirely on peculiar atmospheric con- 

 ditions for development, it is probable that one out of every million 

 will not vegetate the same season in which they are produced, though, 

 like the seeds of some of the larger vegetables, they may, if favour- 

 ably circumstanced, remain dormant for a great length of time, until 

 the conditions essential to their development do occur, when they 

 will appear in extraordinary quantities, as we occasionally find to be 

 the case. It is not well known how the spores gain admission to the 

 vegetable tissue, but Dr. Greville, of Edinburgh, whose knowledge of 

 the fungi is so accurate and extensive, has lately expressed an opi- 

 nion, that they may at all times inhabit the tissue of those species of 

 plants to which they are respectively peculiar, without, under ordi- 



