On the Prevention of Mildew. 



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 congeners, 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. Tf damp and cold weather in July succeed 

 that which has been warm and bright, without the interven- 

 tion 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 mois- 

 ture, 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 con- 

 stantly 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 inter- 

 sected ; 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 organizable matter into the seed vessels 



