LARSEN ON A DISEASE OF THE WHITE BIRCH. 49 



biiim must l»e injui-ed before this giowtli takes place. Between the 

 burrow and the sclerotic tissue a secondary cambium is formed which 

 jj,ives off cork cells towards the burrow tending to fill it up. In some 

 cases this cambium could be connected with the cork cambium of the 

 tree and may take its origin from this. But it was not demonstrated 

 that this was always the case. It may to some extent take its origin 

 from the parenchyma. In any case it seems to give oft" cells both to 

 form the c(U'ky layer mentioned and also in the opposite direction to form 

 the sclerotic tissue. 



The cambium of the tree after a time bridges over the burrow and 

 its covering of sclerotic tissue, forming an arch and performing its 

 normal function covers the Avhole with wood, and while preserving the 

 external ridges buries the burrow deeper and deeper. Burrows, buried 

 in this way, were found with three annual rings above them showing 

 that the trees would overcome the injuries were it not for renewed 

 attacks by numerous beetles. The forming of sclerotic tissue and sec- 

 ondary cambium is probiibly a means to this end. 



That this is true and that the growth mentioned is due to mechanical 

 injury, at least to a large extent, was demonstrated by driving nails 

 through the cambium, leaving wounds which showed the same growth 

 of sclerotic tissue as in the case of the burrows. 



In many cases the holes left by the escape of beetles were filled up 

 by the cork cambium growing down and filling them with cork, even 

 to beyond the chambers. Holes left by the falling of dead twigs are 

 filled in the same way. That the tissues mentioned as cork were really 

 such was shown by testing with strong sulphuric acid. 



Agrilus anxius has been found upon the willows and it may be that 

 the birch is a new host. In that event, it is not so strange that they 

 should thrive; it may take some time before they are followed by 

 their natural enemies. 

 7 



