434 The Blight in Pear Trees. 



of sap wood, and with this means of support, the extremest 

 branches continue green and flourishing. If the destruction of 

 the pith and heart wood, would cause bhght, no hmb, broken 

 beyond the centre, could ever live. But in the propagation 

 of plants by layers, and in the planting of hedges, it is an 

 every day's practice to cut beyond the pith, and even split 

 the limb upwards, and the limb so treated, not only lives, 

 but overcomes the wound, and maintains the successive an- 

 nual growths at the extremity. If the sap wood remains en- 

 tire, as it did in the case described by Mr. Lowell, the limb 

 could not die, although the pith and heart wood should be 

 taken away; even one fourth of the sap wood, if a live bark 

 were adhering to it, would preserve the life of the limb, if the 

 residue had been severed by the knife or by a storm; and to 

 assume that the destruction of the pith and heart wood 

 would cause its death, is not only unsupported by evidence, 

 but is in the face of evidence to the contrary. If the blight 

 in pear trees is caused by an insect, it is caused in some 

 manner not yet described. At present we are bound to main- 

 tain that there is no such thing as insect blight in pear trees. 

 Let me add, however, that there is an injury caused by in- 

 sects, to the growing shoots of the pear, by which the ex- 

 tremity of the shoot droops and dies, but the only effect of 

 this is, to cause the pushing of the next good bud below the 

 injury. This kind of injury is confined to shoots that have 

 not reached the woody state. 



Your fire blights and ours have the same external charac- 

 teristics, according to the descriptions given, and it is fair to 

 conclude, that they are both produced by the same cause. 



The injury is caused by cold, occurring at some period of 

 active growth, when the tree is not prepared for the expos- 

 ure. The effect is not shown by killing the young shoots, 

 for they always put forth their leaves the next year, and this 

 disposes of the surmise about unripened wood. The injury 

 is always to the trunk, or the larger limbs of the tree, and is 

 nearly always confined to a small spot, producing a girdle of 

 dead bark, which is followed by the death of all that part 

 which is above the injury. For, although a tree may, by the 

 heart wood only, put forth its leaves and make long shoots 

 of new wood, it cannot continue to live, without a living 

 passage maintained between the bark and alburnum, from 



