_t_, Scales of primary thallus.]
_Reproduction_.
There are three methods of reproduction of the lichen: by fragmentation,
by soredia, by the formation of fungal spores. In the first process,
portions of thallus containing gonidia may be accidentally separated and
so may start new plants. The second method is only a special process of
fragmentation. The soredia are found in a large number of lichens, and
consist of a single gonidium or groups of gonidia, surrounded by a
sheath and hyphae. They arise usually in the gonidial layer of the
thallus by division of the gonidia and the development around them of
the hyphal investment; their increase in number leads to the rupture of
the enclosing cortical layer and the soredia escape from the thallus as
a powdery mass (fig. 12). Since they are provided with both fungal and
algal elements, they are able to develop directly, under suitable
conditions, into a new thallus. The soredia are the most successful
method of reproduction in lichens, for not only are some forms nearly
always without spore-formation and in others the spores largely
abortive, but in all cases the spore represents only the fungal
component of the thallus, and its success in the development of a new
lichen-thallus depends on the chance meeting, at the time of
germination, with the appropriate algal component.
_Conidia._--Contrary to the behaviour of the non-lichen forming
Ascomycetes the lichen-fungi show very few cases of ordinary conidial
formation. Bornet describes free conidia in _Arnoldia minitula_, and
_Placodium decipiens_ and _Conidia_-formation has been described by
Neubner in the Caliciae.
[Illustration: After Sachs, from De Bary's _Vergleichende Morphologie
und Biologie der Pilze_, _Mycetozoen und Bacterien_, by permission of
Wilhelm Engelmann.