ILLUMINATION OF LICHENS 245 



insertion of the apothecium which is thus well exposed and prominent ; but 

 Zukal' 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 repro- 

 ductive 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= 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 ex- 

 ceptions, and brilliant yellow and white thalli frequently form a conspicuouf 

 feature of vegetation. Black lichens are rare, but occasionally the very darl- 

 brown of foliaceous species such as Gyrophora or of crustaceous species such 

 as Verrucaria ntaura 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, CoUemaceae, 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. Leptognim 

 Burgessii, another thin plant of the same family, is frequently of a purplish 



1 Zukal 1896. ' Hedlund 1892, p. ai. 



