In 1865 De Bary suggested the possibility that such lichens as Collema, Ephebe, &c., arose as a result of the attack of parasitic Ascomycetes upon the algae, Nostoc, Chroococcus, &c. In 1867 the observations of Famintzin and Baranetzky showed that the gonidia, in certain cases, were able to live outside the lichen-thallus, and in the case of Physcia, Evernia and Cladonia were able to form zoospores.
Rees in 1871 produced the sterile thallus of a Collema from its constituents; later Stahl did the same for three species.
Collema) or may be fixed more or less closely to it by special hyphae or rhizoids.
- Section of Homoiomerous Thallus of Collema conglomeratum, with Nostoc threads scattered among the hyphae.
In favour of the conidial view is the fact that in the case of Collema and a few other forms the spermatia have been made to germinate in artificial cultures, and in the case of Calicium parietinum Moller succeeded in producing a spermogonia bearing thallus from a spermatium.
In relation to the view that the spermatia are sexual cells, or at least were primitively so, it must be pointed out that although the actual fusion of the spermatial nucleus with a female nucleus has not been observed, yet in a few cases the spermatia have been seen to fuse with a projecting portion (trichogyne) of the ascogonium, as in Collema and Physcia, and there is very strong circumstantial evidence that fertilization takes place (see later in section on development of ascocarp).
The first exact knowledge as to the origin of the ascocarp was the work of Stahl on Collema in 1877.
The latter :F' s showed that in Collema crispum there are two kinds
Thus ascogonia with trichogynes have been observed in Endocarpon, Collema, Pertusaria, Lecanora, Gyrophora, Parmelia, Ramalina, Physcia, Anaptychia and Cladonia.
In Collema and a form like Xanthoria parietina it is probable that actual fertilization takes place, and possibly also in some of the other forms. It is probable, however, that in the majority of cases the ascogonia develop without normal fertilization, as is necessarily the case where the ascogonia have no trichogynes or the spermatia are absent.
Collema limosum, Peltidea venosa); while many may be found growing on all kinds of soil, from the sands of the sea-shore to the granitic detritus of lofty mountains, with the exception of course of cultivated ground, there being no agrarian lichens.