The South Australian Naturalist. 
63 
' ' occur in the Antarctic, where no other growth, except perhaps a few 
pp algae, is seen; they fade out only as they reach the permanent 
covering of ice and snow. Even in the heat of the desert they 
grow. The hot dry sands of African and Asiatic deserts show 
peculiar forms. For the most part these species are not rooted 
but blow about in the drying winds, quite parched and dry. Their 
existence in such places is due to tlieir power of rapid water ab- 
sorption, and also to their power of entering into a resting stage 
with no apparent preparation. While water is present, growth 
;^.and reproduction proceed hurriedly but when the supply is ex- 
/ hausted they roll up and become dormant. Generally speaking 
[| the best localities for the luxuriant growth of lichens are where 
liJj plenty of light and fresh air, and accasionally some fresh water 
c are present. Perhaps their demand for fresh-water explains their ab- 
sence from towns; it is well known that smoke fumes are toxic 
to them. 
The lichen is not a simple orpnism, its thallus, that it, its 
vegetative body, is compound, being composed of two distinct 
types of organism; a fungus and an alga, living in intimate con- 
nection.^ The fungus forms the main part of the thallus, the algal 
ceils being enclosed in the middle portion by the fungal hyphae. 
The green algal cells are called Gonidia. The species of algae 
which provide the gonidia are simple organisms requiring, norm- 
ally, a great deal of light and moisture. The fungus, on the other 
hand prefers darkness, but still demands much moisture. When 
they enter into partnership they break with all their old ways of 
life, and ask only for light and fresh air with an occasional water 
supply. 
Transverse Section of Lichen Thallus. 
