The Blight in Pear Trees. 15 



describes this shrivelling as preceding the vitiating of the sap, 

 but I do not so find it. The effect produced is equivalent to 

 that caused by an extensive removal of the bark, and a com- 

 plete destruction of the tissue surrounding the wood. The 

 circulation of the elaborated sap, in its descent from the 

 leaves, seems to be thus prevented, and, as the branches be- 

 come exhausted, the blackening of the leaves begins. 



During that summer, (12 July,) I addressed a letter to the 

 Cincinnati Horticultural Society, on the subject of blight, 

 which they caused to be published in the Western Fanner 

 and Gardener for August 1844, the essential parts of which 

 were absorbed into Mr. Beecher's Essay of October, 1844, 

 very much in the same way as the theory of that essay is 

 thought to have been transferred into Mr. Downing's book on 

 Fruits which appeared in the ensuing year. From that let- 

 ter, I make the following extract : — 



"I am thus mirmte in giving the appearance of things pre- 

 sented to my view, because, in my searches for descriptions 

 of the disease called fire blight, 1 find none in the numerous 

 journals I have, and that you may judge whether this be fire 

 blight, as I suppose it is. If this be the disease described by 

 Dr. Mosher, I doubt whether it is caused by the aphis he 

 describes. I find, in my garden, many instances in which 

 the green aphides have destroyed the terminal buds in the 

 young growing shoots, so that the extremities of the shoots 

 wither and die for several inches. The first good bud below, 

 which would otherwise lie dormant, is pushed forward, and 

 becomes the leading shoot. If the brown aphis actually kills 

 the leaf by extracting its juices, would any worse effect ensue 

 than if the leaves were cut off with a knife, which would not 

 kill the limb, unless the succeeding crops of leaves were also 

 removed? I suggest that the disease is, in fact, an infected 

 state of the sap — a fermentation which I cannot think is 

 caused by the brown aphis preying on the petioles of the 

 leaves, and thus preventing a return of the fluids. We are 

 not warranted in supposing that the aphis poisons the sap 

 without some proof; and such a supposition admits that the 

 sap is not all consumed by the insect ; nor are we to assume 

 that the sap is consumed by the aphis, for, if we examine 

 sufficiently early, we shall find it abundant and spoiled — 



