92 REPORT ON THE LICHENS OF EPPING FOREST.
improbable that a lichen-thallus in the leproid condition satisfies
its ordinary requirements, both of nutrition and reproduction :
whether, at long intervals, the production of definite spores,
produced sexually, is necessary to the permanent well-being
of the species, may be surmised, but is not yet demonstrated.
Certain forms (as Lecidea lucida), whose thallus is wholly leprose,
do produce, but infrequently, proper apothecia and spores.
Before giving our list to date, it may be interesting, having
regard to the comparative little that is known by the general
botanist with regard to the natural history and ecology of
Lichens, to make a few remarks on this subject, based mainly
on our own observations in the field, and having reference
chiefly to the conditions which obtain in the district with which
we are here concerned.
HABITATS AND PLANT-ASSOCIATIONS.
We have already noticed, in our first paper, that Epping
Forest is specially rich in the ground forms of Lichens—those
which grow on open heaths. Although our Forest heath-lands
are of but moderate extent, and are, alas, tending to become
less in area each year by the invasive growth of young Birch,
nevertheless Epping Forest presents probably a more extensive
assemblage of heath-dwelling lichens than any other district,
of equal area, within the same radius of the metropolis.
On bare heaths, we find an association of such lichens as
Baeomyces roseus, B. rufus, Pycnothelia papillaria, Cladonia
coccifera, C. macilenta, C. furcata, Cetraria aculeata, Peltigera
spuria, Lecidea granulosa, and (where flints occur, partially
imbedded in the gravelly soil) Rhizocarpon petraeum and Lecidea
crustulata ; together with Polytrichum piliferum and dwarf
heather (Calluna) and Salix repens. Peltigera spuria commonly
establishes itself on burnt patches of earth, the site of fires,
where it keeps company with the moss Funaria hygrometrica.
On wet heaths, amongst heather and Erica tetralix, we get
Cladina sylvatica form tenuis, Cladonia fimbriata sub-sp. fibula,
together with Hypnum schreberi, Polytrichum communis, Dicra-
num scoparium, etc.
The tree-dwelling lichens of Epping Forest are not so flourish-
ing as the ground forms, except in the case of a comparatively
few dominant forms which crowd out all competitors. We