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ILLUMINATION OF LICHENS 245
insertion of the apothecium which is thus well exposed and prominent; but
Zukal 1
sees in this formation an adaptation to enable the frond to avoid
the shade cast by the apothecium which may exceed it in width. In most
lichens, however, and /especially in shade or semi-shade species, the reproductive
organs are to be found in the best-lighted positions.
b. INFLUENCE OF LIGHT ON COLOUR OF FRUITS. Lichen-acids are
secreted freely in the apothecium from the tips of the paraphyses which give
the colour to the disc, and as acid-formation is furthered by the sun's rays,
the well-lighted fruits are always deeper in hue. The most familiar examples
are the bright-yellow species that are rich in chrysophanic acid (parietin).
Hedlund 2 has recorded several instances of varying colour in species of
Micarea (Biatorina, etc.) in which very dark apothecia became paler in the
shade. He also cites the case of two crustaceous species, Lecidea helvola and
L. sulphnrella, which have white apothecia in the shade, but are darker in
colour when strongly lighted.
V. COLOUR OF LICHENS
The thalli of many lichens, more especially of those associated with blue-
green gonidia, are hygroscopic, and it frequently happens that any addition
of moisture affects the colour by causing the gelatinous cell-walls to swell,
thus rendering the tissues more transparent and the green colour of the
gonidia more evident. As a general rule it is the dry state of the plant that
is referred to in any discussion of colour.
In the large majority of species the colouring is of a subdued tone soft
bluish-grey or ash-grey predominating. There are, ho\vever, striking ex-
ceptions, and brilliant yellow and white thalli frequently form a conspicuous
feature of vegetation. Black lichens are rare, but occasionally the very dart
brown of foliaceous species such as Gyrophora or of crustaceous species such
as Verrncaria maura or Buellia atrata deepens to the more sombre hue.
A. ORIGIN OF LICHEN-COLOURING
The colours of lichens may be traced to several different causes.
a. COLOUR GIVEN BY THE ALGAL CONSTITUENT. As examples may
be cited most of the gelatinous lichens, Ephebaceae, Collemaceae, etc. which
owe, as in Collema, their dark olivaceous-green appearance, when somewhat
moist, to the enclosed dark -green gonidia, and their black colour, when dry,
to the loss of transparency. When the thallus is of a thin texture as in
Collema nigrescens, the olivaceous hue may remain constant. Leptogiutn
Burgessii, another thin plant of the same family, is frequently of a purplish
1 Zukal 1896.
2 Hedlund 1892, p. 11.