206 ON THE PREVENTION OF MILDEW IN PARTICULAR CASES. 



Neither the mildew of wheat, nor any other kind, can however I 

 think, be communicated from the leaves and stems of one plant imme- 

 diately to those of another : very numerous attempts made by myself to 

 succeed in experiments of this kind having, I believe, proved wholly 

 abortive ; though I once fancied that I had succeeded in two or three 

 instances. I am, therefore, much inclined to believe that the parasitical 

 fungus, which occasions every disease of this kind, enters the plant, in the 

 first instance, by its roots, and though it may probably be transferred 

 with the graft, and possibly by a bud, from one fruit-tree to another; and 

 if the seeds be capable, like those of many other plants, of remaining 

 sound a considerable time beneath the soil, or in other situations, till 

 circumstances, which are favourable to their growth, occur, the abundant 

 appearance of the mildew, or mushrooms, may be accounted for without 

 supposing them to be generated wholly by the bodies from which they 

 immediately spring. 



I shall not trespass upon the time of the Horticultural Society by 

 dwelling longer upon the primary cause of the various diseases which are 

 comprehended under the name of mildew ; but shall proceed to the 

 immediate object of the present memoir, which is to point out the means 

 by which the injurious effects of the common white mildew may be, in 

 particular cases, prevented. 



The secondary and immediate causes of this disease, and of its con- 

 geners, have long appeared to me to be the want of a sufficient supply of 

 moisture from the soil with excess of humidity in the air, particularly 

 if the plants be exposed to a temperature below that to which they have 

 been accustomed. If damp and cold weather in July succeed that which 

 has been warm and bright, without the intervention of sufficient rain to 

 moisten the ground to some depth, the wheat crop is generally much 

 injured by mildew. I suspect that, in such cases, an injurious absorption 

 of moisture, by the leaves and stems of the wheat plants, takes place ; 

 and I have proved, that under similar circumstances much water will be 

 absorbed by the leaves of trees, and carried downwards through their 

 alburnous substance ; though it is certainly through this substance that 

 the sap rises under other circumstances. If a branch be taken from a 

 tree when its leaves are mature, and one leaf be kept constantly wet, that 

 leaf will absorb moisture and supply another leaf below it upon the 

 branch, even though all communication between them through the bark 

 be intersected ; and if a similar absorption takes place in the straws 

 of wheat, or the stems of other plants, and a retrograde motion of the 

 fluids be produced, I conceive that the ascent of the true sap or organ- 



