DAMAGE CAUSED BY PLANTS. 77 



formation of small trenches so as to isolate the parts of seed-beds 

 infected. 



3. Fungi in the Stem or Branches (in the bark or in the wood). 



Canker of the Pine (Aecidium (Peridernium) pini, var. corticola), 

 often very conspicuous on young trees of Scots, Weymouth, and 

 other species of Pines, on account of the semi-spherical or oval 

 pustules filled with reddish-yellow spores, occasions the inspissation 

 of resin and the formation of pockets of it inside the stem, owing 

 to the action of the mycelium. In consequence of this the growth 

 of the tree ceases at these places ; and when this occurs to any 

 great extent, the whole of the tree above the infected part ceases 

 its vital functions. The dying off of the tops of old Pine trees 

 is very often caused by this fungus. 



The Silver Fir fungus (Aecidium elatinum) occasions the 

 peculiar cankerous swellings that are not infrequently indeed in 

 some localities very frequently noticeable at a greater or less 

 height up the stem in crops of Silver Fir, mostly encircling the 

 bole, and known as Canker of the Silver Fir. The bark gradually 

 dies off from the excrescences formed, often of very considerable 

 size, and the wood is laid bare and begins to rot. This process is 

 stimulated and hastened by the advent of other fungous spores 

 (Polyporus) ; according to the place where the canker occurs and 

 | two or three cankerous places may sometimes be seen on one tree > 

 the bole becomes more or less unsuited for technical purposes, and 

 under any circumstances loses considerably in value. Such stems 

 often break at the diseased parts during storms. 



Along with this cankerous disease there also very frequently 

 occur yellowish-green, loranthus-like excrescences or deformities, 

 like twig-clusters, on the branches of the Silver Fir, (called " witches- 

 I'brooms" in Germany), which are likewise occasioned by Aecidium 

 elatinum, although the connection of these two different forms of 

 disease arising from the same fungus has not yet been satisfactorily 

 explained. In the latter case the spores are developed on the 

 lower side of the needles, and the reproduction of the fungus, and 

 later on the formation of canker, is the result not directly, 

 i however, but by the assistance of some host or plant acting as go- 

 between, not yet discovered and identified. The spores are never 

 (developed at the cankerous place itself. 



