34 Insects Injurious to the Elm. 



of many other insects injurious to the elm, both in its healthy 

 and partially- decayed state. 



Having examined the habits of certain xylophagous, or 

 wood-eating- insects, and having probably arrived at the con- 

 clusion that the ravages of the Scolytus and his congeners have 

 a more direct connection with the decay of elm-trees than 

 increased drainage, or any other cause, it is time to consider 

 the seemingly dangerous method of cure proposed and prac- 

 tised by M. Robert, and to ascertain the principles upon 

 which it must have been adopted, and upon the applicability of 

 which its success must depend. 



The sap, the basis of which is mere rain-water, that is to 

 say water impregnated with carbonic acid, is taken up by the 

 roots, and ascends between the solid wood and the bark, 

 causing the formation of a coating of new wood all round the 

 trunk, which coating, in its soft state, is termed alburnum. It 

 is again through this alburnum that the surplus sap, vitiated by 

 the functions it has performed, has to descend, eventually escap- 

 ing through the spongeoles, or fine fibres of the roots, into the 

 earth. Now, if the ascent of the nutritious moisture be im- 

 peded by the scoring of the Scolyti, or still worse, if, in its 

 vitiated state, its descent be impeded, and its escape prevented, 

 the most fatal consequences must necessarily ensue in some of 

 the forms caused by the morbid retention of a poison. If, 

 therefore, M. Robert can remove the bark, and with it the 

 Scolyti, giving to the alburnum (relieved by his process from 

 the further injury of its enemies) the opportunity of exerting 

 its reparatory functions, which he asserts that it is able to do 

 even when deprived of its natural protection, the external bark, 

 which, he assumes, it is able to restore, then M. Robert appears 

 to have hit upon a mode of cure which, under favourable cir- 

 cumstances, may prove successful. It is, of course, necessary 

 that in removing the bark care be taken to spare in every way 

 the alburnum. My neighbour Dr. Evans informs me that 

 a goat in his garden had eaten all the baric from the lower 

 part of the trunk of a tree to which it was attached, but that 

 not having seriously injured the alburnum the reparatory 

 powers of that substance not only repaired its own injuries, 

 but reclothed itself with a coating of bark. And thus it is seen, 

 in <c ringing " operations, pursued for the purposes of destroy- 

 ing trees, that the alburnum itself must be cut through, as well 

 as the outer and inner bark, and then, no doubt, in the great 

 majority of cases the tree invariably dies. 



It may be stated, in support of M. Robert's theory, that 

 the bark is, speaking by analogy, the bone, rather the shin 

 of the tree, and bone, as is well known, is the result of a kind 

 of organic action which has to an unusual extent the power of 



