In Ramalina the frond frequently turns at a sharp angle at the point of insertion of the apothecium which is thus well exposed and prominent; but Zukal[882] 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[883] 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. sulphurella, 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, however, striking exceptions, and brilliant yellow and white thalli frequently form a conspicuous feature of vegetation. Black lichens are rare, but occasionally the very dark brown of foliaceous species such as Gyrophora or of crustaceous species such as Verrucaria 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. Leptogium Burgessii, another thin plant of the same family, is frequently of a purplish hue owing to the purple colour of the gonidial Nostoc cells. The dull-grey crustaceous thallus of the Pannariaceae becomes more or less blue-green when moistened, and the same change has been observed in the Hymenolichens, Cora, etc.
In Coenogonium, the alga is some species of Trentepohlia, a filamentous genus mostly yellow, which often gives its colour to the slender lichen filaments, the covering hyphae being very scanty. Other filamentous species, such as Usnea barbata, etc., are persistently greenish from the bright-green Protococcaceous cells lying near the surface of the thalline strands. Many of the furfuraceous lichens are greenish from the same cause, especially when moist, as are also the larger lichens, Physcia ciliaris, Stereocaulons, Cladonias and others.
b. Colour due to Lichen-Acids. These substances, so characteristic of lichens, are excreted from the hyphae, and lie in crystals on the outer walls; they are generally most plentiful on exposed tissues such as the cortex of the upper surface or the discs of the apothecia. Many of these crystals are colourless and are without visible effect, except in sometimes whitening the surface, strikingly exemplified in Thamnolia vermicularis[884]; but others are very brightly coloured. These latter belong to two chemical groups and are found in widely separated lichens[885]: