LICHENOLOGY OF ICELAND 149 
that they die away at their base make them unfit for life on the 
bark of trees and on stones, and consequently they do not occur 
there. 
Some fuller data concerning the biology of the fruticose lichens 
will be given here: — 
The hypothallus is the purely mycelial lichen-tissue, free of 
gonidia, which is formed by the germination of the spore (the so- 
redium or perhaps the pycnoconidium). It has been observed in 
all the species of the genus Cladonia, and in Stereocaulon conden- 
salum; but as to all the other species of the latter genus it has, in 
some, never been observed, and in others, is very insignificant, and 
is then, only for a time, of importance to the life of the species, 
as it dies away early. It lives long in all hypothallus-wanderers, 
and constitutes — as suggested by the name — their only means 
of wandering. On the other hand, it disappears very early in the 
primary-scale wanderers, and in the majority of the podetium- 
wanderers. | 
It is always formed of very loosely woven hyphæ, which grow 
centrifugally from the germination-centre. Wherever green algæ 
suitable for the species, is encountered by it on the surface of the 
ground, it weaves its hyphæ round them, and forms thereby the 
first beginning either of primary scales (Cladonia) or of direct po- 
detia (Stereocaulon). This process has, as regards Cladonia, been 
described by Krabbe (1891) and Wainio (1898). 
Some of the hypothallal hyphæ are often formed as fairly thick, 
dark hyphal bundles, almost devoid of intercellular spaces, especi- 
ally where they are continued up into the base of the primary scale 
(Cladonia cornuta, C. verticillata). The hyphæ easily come into con- 
tact with mineral-grains, humus-particles, plant-remains with their 
structure still intact, and earth algæ. About this the following is to 
be noted: — 
Mineral-grains, especially sand-grains, adhere to the hyphæ of 
several species. I believe that this happens through the cell-walls 
being covered with a slight (microscopically-invisible) covering of 
mucus. The sand-grains themselves are always finely striated on 
the surface, no doubt from weathering, for it cannot be proved that 
the hyphæ exercise any chemical influence upon them, and we 
must be careful not to state definitely that the roughnesses are marks 
of corrosion. 
The humus-particles are opaque under the microscope. Where 
