20 The Blight in Pear Trees. 



Now Mr. Lowell had somewhat predetermined the case, 

 that the bhght was caused by insects. So patient a search as 

 he made could not fail to be rewarded with the discovery of 

 insects. Forsyth had previously written that " in this dis- 

 ease, the perspiring matter is rendered thick and glutinous, 

 and so becomes food for insects, which are always found in 

 vast numbers on fruit trees that are affected with this kind of 

 blight;" and he adds "these insects, however, are not the 

 original cause, as some imagine, but the natural consequence 

 of blight." 



But does Mr. Lowell's description of the depredation by this 

 insect, — the Scolytus Pyri, — warrant the conclusion, that it 

 could cause the death of the limb? It does not warrant it, 

 according to any known theory of vegetation in woody plants. 

 The eating out of the pith, and all the heart- wood, would not 

 kill the limb, for the sap would still ascend through the al- 

 burnum, though in diminished quantity. He did not state 

 that the alburnum was also eaten through, and, of course, he 

 did not mean to state it, for, if that were the case, the limb 

 would break, and so be killed in a very simple manner. Mr. 

 Downing says that the insect saws off " a considerable portion 

 of the vessels, which convey the ascending sap, at the very 

 period, when the rapid growth of the leaves calls for the larg- 

 est supply of fluid from the roots ; the growth and vitality of 

 the branch are checked, and finally extinguished." To this, 

 it may be replied, that the facts are furnished by conjecture. 

 Mr. Lowell found the insect ready to emerge in July, and, 

 being then perfect, it would probably proceed to deposit its 

 eggs. Whether the larva eats its way down, in autumn, or 

 in the ensuing spring, is not stated. If in autumn, and the 

 excision was great enough to produce the effect contended for, 

 the branch would not have grown at all ; if it were in the 

 spring, the process must have been gradual, and the external 

 signs of it would be also gradual, by enfeebled growth. At 

 that time also, the leaves are already grown, and a check in 

 the supply would only hasten a ripening of the leaves, and 

 diminish the quantity of the new wood deposited by the de- 

 scending sap. The checking of the supply would not vitk " 

 the condition of the eliminated sap in its descent from the 

 leaves ; and the sap is always found vitiated and discolored. 



