208 ON THE PREVENTION OF MILDEW IN PARTICULAR CASES. 



of the peach to become a very formidable enemy. Where, on the con- 

 trary, a deep and fertile dry loam permits the roots to extend to their 

 proper depth ; and where the situation is not so low as to be much 

 infested with fogs, I have found little of this disease : and in a forcing- 

 house I have found it equally easy, by appropriate management, to intro- 

 duce or prevent the appearance of it. When I have kept the mould 

 very dry, and the air in the house damp and unchanged, the plants have 

 soon become mildewed ; but when the mould has been regularly, and 

 rather abundantly watered, not a vestige of the disease has appeared. 



It must be confessed that it is not easy to account, at first view, for 

 the appearance of this disease under some of the preceding and various 

 other circumstances, if it be produced by a parasitical plant which 

 propagates by seeds ; but all we ever see of the mildew is simply its 

 fructification : the plant itself, if it be one, is wholly concealed from our 

 senses ; and it may consequently be transferred from one plant to another 

 by the graft or bud, and never become visible till the health of the tree 

 become affected by other causes. I could state some cases which are 

 very favourable to this opinion, for this disease appears readily to be 

 communicated by a graft to another tree, when that grows in the same 

 soil, and in similar external circumstances. The different species of 

 minute insects which feed upon the bodies of our domestic cattle are 

 scarcely ever seen, and never injurious so long as the larger animals 

 retain their health and vigour ; but when these become reduced by 

 famine or disease, the insects multiply with enormous rapidity, and 

 though they are at first only symptomatic of disease, they ultimately 

 become the chief and primary cause of its continuance. The reciprocal 

 operation of the larger plant and the mildew upon each other may 

 possibly be somewhat similar. 



I offer the preceding opinions merely as conjectures : the hypothesis I 

 have chosen has led me to the successful treatment of the disease in 

 particular cases, and it may in the same way lead others : and I therefore 

 venture to submit it to the consideration of the Horticultural Society 

 without being very confident of its truth. If, however, the countless 

 millions of apparently organised bodies, which are generated by the 

 different species of fungus, be not seeds, nature appears to wander 

 widely from its ordinary path : for amidst all its boundless profusion 

 and exuberance, it does not ever, in other cases, appear to labour wholly 

 in vain. 



P.S. Observing that the almond- trees, round the metropolis, are 

 likely to produce a considerable crop in the present year, I wish to 



