96 REPORT ON THE LICHENS OF EPPING FOREST.
these extreme conditions, the lichen, while not actually succumb-
ing to its inclement environment, is compelled to suspend its
growth until the return of more favourable seasons ; and so its
rate of growth is slow. In moist warm climates, where lichens
attain their greatest development, external conditions are
favourable to much more rapid development ; but here, un-
fortunately, we have as yet no data as to rate of growth. But
even in our own temperate latitudes it is evident that the
extremely slow development claimed for lichens does not always
hold. When we meet with fruiting lichens, overspreading
decaying leaves, which can scarcely have lain on the ground
more than two or three years, others growing on old boots, or
on clung, and fruiting freely, others overspreading growing
mosses ; and when we notice, as we do sometimes, that on an
old wall the best developed lichens are growing, not on the wall
itself, but on the cement jointing which was added when the
wall was last repaired, perhaps 20 years before ; we begin to
suspect that, after all, under more equable conditions of tem-
perature, and in a humid atmosphere, the rate of growth of
some lichens at least must be far more rapid than has been
generally assumed to be the case. We know one mass of con-
crete at Theydon Bois, surrounding a surface-water drain
which was formed in 1903, and which, in 1910, after only seven
years, was covered with Lecanora galactina in abundant fruit ;
and a Portland stone garden-ornament, new in 1904, was, in
1910, after only six years, covered with patches of a fruiting
Verrucaria (probably nigrescens). It is perhaps noteworthy
that these forms have always a very scanty development of
thallus and produce their fruit very freely.
ENEMIES AND PROTECTIVE RESEMBLANCE.
We have now to enquire what are the enemies with which
Lichens have to contend in our Forest.
Mites of the family Oribatida must be reckoned among the
chief foes of these plants, upon which they feed, seeming to have
a special predilection for the ripe fruits. We have had excellent
specimens of Physcia parietina spoiled by hidden mites of this
family, which have eaten out the contents of the mature apothecia
after the lichens have been gathered. One can sometimes
see small flocks of the mites browsing upon the thallus of tree-