THE FUNGI 171 
organism rather than to endeavour to extir- 
pate it afterwards. 
Some of these destructive fungi, instead of 
only attacking the dead tissues—the wood— 
of the trees, invade and kill the living cells. 
This is a more serious matter, for we must 
remember that only a very small part of a 
trunk is really alive in the strict sense of the 
word, that is, contains living protoplasm. 
Any pest which attacks the living tissues, and 
especially the cambium, often speedily kills 
it, or at any rate renders it practically worth- 
less. Such fungi are those which produce 
the larch canker (Dasyscypha Willkommii) and 
the beech canker (Nectria ditissima), the 
latter being especially destructive to the cells 
of the cambial region and thus producing 
very dangerous lesions. 
One of the most interesting of the tree 
diseases is that produced by the fungus known 
as Armillaria mellea. The fructifications are 
easily recognised as clusters of brown toad- 
stool-like bodies which spring from the roots 
or stumps of dead trees at the ground level. 
Each “toadstool” is characterised by the 
possession of a ring or frill underneath the 
cap which bears the gills. Before its life 
~ history became known, and _ consequently 
methods could be devised to check its progress, 
the fungus was a very dangerous one, especi- 
ally when it invaded the pine woods, for it 
spreads fairly rapidly from one tree to 
another. The mycelium grows in the cambial 
