_Gonidia._--It has been made clear above that the gonidia are nothing
more than algal cells, which have been ensnared by fungal hyphae and
made to develop in captivity (fig. 1). Funfstuck gives ten free living
algae which have been identified as the gonidia of lichens.
_Pleurococcus_ (_Cystococcus_) _humicola_ in the majority of lichens,
e.g. _Usnea_, _Cladonia_, _Physcia_, _Parmelia_, _Calicium_, many
species of _Lecidea_, &c., _Trentepohlia_ (_Chroolepus_) _umbrina_ in
many species of _Verrucaria_, _Graphidieae_ and _Lecidea_; _Palmella
botryoides_ in _Epigloea_; _Pleurococcus vulgaris_ in Acarospora,
Dermatocarpon, Catillaria; _Dactylococcus infusionum_ in _Solorina_,
_Nephromia_; _Nostoc lichenoides_ in most of the Collemaceae; _Rivularia
rutida_ in _Omphalaria_; _Lichina_, &c., _Polycoccus punctiformis_ in
_Peltigera_, _Pannaria_ and _Stictina_; _Gloeocapsa polydermatica_ in
_Baeomyces_ and _Omphalaria_; _Sirosiphon pulvinatus_ in _Ephebe
pubescens_. The majority of lichens are confined to one particular kind
of gonidium (i.e. species of alga) but a few forms are known (_Lecanora
granatina_, _Solorina crocea_) which make use of more than one kind in
their development. In the case of _Solorina_, for example, the principal
alga is a green alga, one of the Palmellaceae, but _Nostoc_ (a
blue-green alga) is also found playing a subsidiary part as gonidia. In
L. _granatina_ the primary alga is _Pleurococcus_, the secondary,
_Gleococapsa_.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.