INTRODUCTION Xix 
or less advantage to each member or symbiont of the dual organism. 
The alga undoubtedly grows with great vigour in the lichen- 
thallus, being excited to increased vitality by contact with the 
fungus, or by conditions, such as increased moisture, supply of 
inorganic salts, shelter, etc., that are specially favourable to its 
growth. On the other hand, the fungus withdraws from the 
alga the necessary carbohydrates. The presence of hyphe within 
the algal cells observed in certain lichens, and of empty algal 
membranes in others, testify to the occasional harmful ravages of 
the fungus ; but any theory of lichens as merely parasitic fungi 
is incompatible with the continuous healthy development of the 
lichen plant. 
Excretory products——In many lichens there is an abundant 
formation of oxalate of lime; it is laid down in crystals on the 
outside of the hyphe, medullary or cortical. In Lecanora 
esculenta, a limestone desert lichen, it has been proved that the 
crystals form sixty per cent. or more of the dry substance of the 
thallus. 
There is also an enormous production of lichen acids, organic 
products of varying chemical formule found only in lichens, and 
due undoubtedly to the peculiar symbiotic relationship of fungus 
and alga. Among the best known are parietin (chrysophanic 
acid), which gives the brilliant yellow colour to several lichens, 
and the valuable series of acids that produce the orchill of 
commerce, called also cudbear and litmus. Orchill provides a 
beautiful purple dye, and though it can be extracted from a 
number of lichens, the chief commercial source is Roccella tine- 
toria, which grows in great abundance on the rocky coasts of 
southern Europe. The crustaceous lichen Lecanora tartarea also 
yields a purple dye. 
ECOLOGY AND DISTRIBUTION. 
Lichens are xerophytic in structure and well-adapted by 
their composite nature to withstand extreme drought and extreme 
weather conditions. In some cases water may be absorbed from 
the soil, but in most lichens the habitat precludes that possi- 
bility : they live nearly always in xerophytic conditions—on the 
bark of trees, on the bare rock, on acid moorland soil, on sandy 
wastes or by the seashore. The hyphe swell up and retain for 
a long time the moisture they receive mostly from mist or rain. 
There is no regular provision for transpiration except in a few 
