1 90 DISEASES OF TREES 



wood the mycelium, which appears as a small cushion between 

 the scales, grows into the cortex of the neighbouring tree. It is 

 easy to induce infection artificially by taking a piece of cortex 

 containing a portion of living' mycelium still capable of growth, 

 and binding it firmly to the cortex of the root of another tree. 



Owing to the mycelium spreading subterraneously from tree 

 to tree, these well-known gaps, which increase in size each year 

 by the death of the marginal trees, appear in woods. At one 

 time no reason whatever could be assigned for the peculiar 

 behaviour of these gaps. On account of the decomposition of 

 the wood proceeding rapidly and advancing far up the stem, and 



as it is succeeded by the 

 death of the tree, the dis- 

 ease is to be classed with 

 the most dangerous forms 

 of "red-rot." It is very 



FIG. 1 2 1. -The thinner root has been killed by abundant in the pine 

 T. radiciperda, and the stronger one has forests of North Germany,* 

 been infected at the point of contact. The . . 



disease has spread as far as the dark shading, and quite as much SO in 

 One eighth natural size. the spruce woodS) especi- 



ally where these are situated 



in hilly districts. There is this difference to be noted, however, 

 that when pines are killed by the parasite it is usually only their 

 roots that are affected and rotten, the stem, with the exception 

 of the stool, showing no signs of decomposition. The wood of 

 the stool is generally strongly impregnated with resin, and I 

 believe I am right in concluding that it is the abundant resinous 

 contents of the pine, which are especially prominent in the lower 

 part of the stem, that form a barrier to the upward growth of the 

 mycelium of the fungus. In the case of the spruce, on the other 

 hand, and of the Weymouth pine, which is poor in resin, decom- 

 position of the wood spreads high into the stem. 



It appears to be necessary to keep a watchful eye upon 

 coniferous woods at all stages of their growth, so that diseased or 

 dead trees may be instantly removed. In the case of the older 

 woods, one may isolate the diseased spots by surrounding them 

 with narrow trenches, and by severing all roots that may be 

 encountered. In order to attain this end we must, of course, 

 * [It is also by no means uncommon in this country. ED.] 



