By Thomas Andrew Knight, Esq. 



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 mul- 

 tiply 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 treat- 

 ment 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 organized bodies, which are generated 

 by the different species of fungus, be not seeds, nature 



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