48 DISEASES OF TREES 



from branch to branch. Thus, when a disease spreads in a wood 

 in this way, it does so with relative slowness, but, in dense woods 

 at least, it is, as a rule, characterised by the death of all or most 

 of the trees inside the local area of distribution. The result is 

 that blanks, varying in size, gradually occur in the wood. 



In the case of Trametes radiciperda, the most dangerous enemy 

 of spruce and pine plantations, contact of the diseased root 

 containing the fungus with the sound root of a neighbouring tree 

 is necessary in order that the latter may be penetrated by the 

 mycelium which protrudes from between the bark-scales. In the 

 case of Agaricus melleus mycelial strands, in the form of rhizo- 

 morphs, spring from the diseased rpots, and proceed to spread 

 underneath the surface of the ground. The roots of sound 

 conifers that are encountered are embraced, and an entrance is 

 effected between the bark-scales : these are forced off by means 

 of the conical apices of the rhizomorphs, which then bore into 

 the living tissues. 



In the case of Rosellinia qiiercina, which destroys the roots of 

 the oak, the delicate filiform mycelium which here and there 

 forms rhizoctones spreads during moist warm weather from the 

 diseased plant into the upper layers of the soil, where it attacks 

 and destroys the roots of neighbouring plants in a manner which 

 will be described more fully later on. On account of the 

 mycelium being capable of forming small round sclerotia on oak- 

 roots, and of assuming a resting condition, the parasite is after- 

 wards enabled to resume the growth which has been interrupted 

 by such unfavourable conditions as cold or a temporary lack of 

 moisture in the soil. 



Dematophora necatrix spreads in a similar manner in vine- 

 yards. 



The distribution of a parasite by spores and gonidia is not, as 

 in the case of infection by the mycelium, confined to plants in the 

 immediate neighbourhood, although these are certainly most 

 exposed to the danger of infection. It may, in fact, happen 

 that trees at a great distance are infected, while those in the 

 immediate neighbourhood remain sound. When treating of 

 special cases we shall have occasion to bring into prominence 

 how various are the conditions that influence this question, and, 

 in particular, how animals and men, by spreading the spores, may 



