LICHENOLOGY OF ICELAND 159 
to Friederich, these gonidia-areas have come into existence in 
places where the gonidia (algæ) have accidentally fallen on the 
lichen-mycelium. According to Stahlecker each area has originally 
been an independent thallus, which, by coming into contact with 
similar neighbouring thalli, forms with these a “Gesamtthallus,” 
which may afterwards grow as a unity, starting from a common 
centre. This interpretation sounds quite incredible, and I think it 
is very rarely, if ever, in accordance with fact. Can it, on the whole, 
be understood that these smaller thalli are “independent,” as they 
have all been produced by the same lichen-mycelium ? 
It is quite another question, whether a group of really indepen- 
dent thalli, produced each from its own ascospore, on meeting, can 
alter and carry on a joint growth. About this nothing is known, à 
priori, it does not seem very probable. 
In reality, these small thallus-patches containing gonidia, men- 
tioned by Stahlecker, must quite naturally be regarded as ana- 
logous, for instance, to the primary scales in Cladonia, which are 
also small green gonidia-containing thalli on a common mycelium; 
or with the exactly corresponding balls of gonidia in numerous 
crustaceous earth-lichens (Lecidea alpestris, L. uliginosus, etc.). 
Quite another separation into patches may moreover take place 
by existing patches splitting asunder into separate parts by growth- 
tensions (or by drying?) (see “Dan. Lik. Ok.,” fig. 19, a, b, c, d). 
When the thallus is smooth and non-partitioned, Stahlecker 
is of opinion that it is an old, formerly partitioned thallus. I can- 
not believe this interpretation of the condition. 
Friederich has found the gonidia-layer of the silicicolous 
lichens to be thicker than that of the calcareous lichens, Fünf- 
stick has also found this to be the case. 
The mode of propagation has been investigated by Beckmann, 
who found that some species (Lecanora badia, L. cenisea), the thalli 
of which are partitioned, may reproduce by means of detached 
portions of the thallus, whereas soredia are absent. On the other 
hand, the partitioned thalli of the Rhizocarpon spp. do not appear 
to be able to reproduce in this way. 
Thin, cohering (non-partitioned) thalli do not appear to be able 
to reproduce in this way. Whether this mode of propagation, on 
the whole, plays any important part in nature, compared with pro- 
pagation by spores, I regard as doubtful. 
With regard to capacity for competition, the crustaceous lichens 
