LICHENOLOGY OF ICELAND 145 
gradually to the substratum, as they come in contact with it. How 
they attach themselves, and how far they are of any other import- 
ance than to fix the plant in the substratum, is not known. The 
thallus itself is always dorsiventral and in some species it dies away 
in the middle, its single lobes thus becoming isolated. Zukal (1895) 
has shown that several of the earth-lichens “wander” by a kind of 
mycelium, which proceeds from their rhizines, and run horizontally 
below the surface of the ground, forming new thalli here and there, 
as in Peltigera venosa and Solorina saccata. This mode of propaga- 
tion corresponds exactly with that by which crustaceous lichens 
with a mycelium of radiating, centrifugal growth, form numerous 
small balls of gonidia, which by their abundance fuse into a granu- 
lose thallus; or with that by which Cladonia forms its centrifugally- 
growing scales (primary thallus) or Sfereocaulon its scales which 
afterwards develop into podetia (Danske Likeners Økologi, fig. 91); 
it is no doubt the most natural explanation of the fact, that the 
form of many foliaceous lichens is that the thallus consists of one 
or more lobes, which have a base on the surface of the earth itself, 
and grow from thence unilaterally forward away from that base. 
The Erect foliaceous lichens are, although erect, very com- 
monly dorsiventral. So far as my investigations go, they die away 
below (the spot corresponding with the centre of the procumbent 
lichens) and keep on growing at the apex. They escape being blown 
away by being fastened by their “haptera” each to the other or to 
other things (Sernander, 1901). These haptera, which have not been 
investigated more closely, have been found by Sernander in Ce- 
traria islandica, cucullata, hiascens and nivalis, and are transformed 
cilia, which, as is well-known, are extremely common in this genus. 
The means of propagation in the foliaceous earth lichens appear 
to be ascospores, or perhaps pycnoconidia. On the other hand, 
soredia and detached portions of thallus do not appear to play any 
part in the dispersal of these species either. Otherwise, the whole 
class has been as yet very little investigated. It is evident that pro- 
cumbent foliaceous lichens are weak competitors, and are easily 
covered by other plants. As regards abundance of individuals they 
also play but a slight rôle in nature. They have far better chances 
on stones and trees, and are very common in such stations. The 
erect foliaceous lichens have far greater advantages in competition, 
and are much richer in individuals than are the others. 
Iceland has the following foliaceous earth lichens: — 
The Botany of Iceland. Vol. II. 10 
