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ILLUMINATION OF LICHENS 243
shade it assumes a grey-green hue and yields only small quantities of
parietin. Placodium elegans, normally a brightly coloured yellow lichen,
becomes, in the strong light of the high Alps, a deep orange-red. Rhizo-
carpon geographicum is a vivid citrine-yellow on high mountains, but is
almost green at lesser elevations.
c. SHADE LICHENS. Many species grow where the light is abundant
though diffuse. Those on tree-trunks rarely receive direct illumination and
may be generally included among shade-plants. Wiesner found that corticolous
forms of Parmelia saxatilis grew best with an illumination between
and y^ of full
light, and Pertusaria amara from ^ to ^j both of them could
thrive from ^ to 3^, but were never observed on trees in direct light. Physcia
ciliaris, which inhabits the trunks of old trees, is also a plant that prefers
diffuse light. In warm tropical regions, lichens are mostly shade-plants:
Wiesner records an instance of a species found on the aerial roots of a tree
with an illumination of only -^.
In a study of subterranean plants, Maheu 1
takes note of the lichens that
he found growing in limestone caves, in hollows and clefts of the rocks, etc.
A fair number grew well just within the opening of the caves; but species
such as Cl. cervicornis, Placodium murorum and Xanthoria parietina ceased
abruptly where the solar rays failed. Only a few individuals of one or two
species were found to remain normal in semi-darkness: Opegrapha hapalea
and Verrucaria muralis were found at the bottom of a cave with the thallus
only slightly reduced. The nature of the substratum in these cases must
however also be taken into account, as well as the light influences: lime-
stone for instance is a more favourable habitat than gypsum the ; latter, being
more readily soluble, provides a less permanent support.
Maheu has recorded observations on growth in its relation to light in
the case of a number of lichens growing in caves.
Physcia obscura grew in almost total darkness; Placodium murorum
within the cave had lost nearly all colour; Placodium variabile var. deep
within the cave, sterile; Opegrapha endoleuca in partial obscurity; Verrucaria
rupestris f. in total obscurity, the thallus much reduced and sterile; Verrucaria
rupestris in partial obscurity, the asci empty; Homodium (Collema}
granuliferum in the inmost recess of the cave, sterile, and the hyphae more
spongy than in the open.
Siliceous rocks in darkness were still more barren, but a few odd lichens
were collected from sandstone in various caves : Cladonia squamosa, Parmelia
perlata var. ciliata, Diploschistes scruposus, Lecidea grisella, Collema nigrescens
and Leptogium lacerum.
d. VARYING SHADE CONDITIONS. It has been frequently observed
that on the trees of open park lands lichens are more abundant on the side
1 Maheu 1906.
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