When the fungus is grown elsewhere than in the ants' nest it produces gonidia instead of the white masses on which the ants feed, hence it seems that these masses are indeed produced as the result of some unknown cultural process.
The green (or blue-green) cells were termed gonidia by Wallroth, who looked upon them as asexual reproductive cells, but when it was later realized that they were not reproductive elements they were considered as mere outgrowths of the hyphae of the thallus which had developed chlorophyll.
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.
In 1869 Schwendener put forward the really illuminating view - exactly opposite to that of Baranetzkythat the gonidia in all cases were algae which had been attacked by parasitic fungi.
Although Schwendener supported this view of the " dual " nature of lichens by very strong evidence and identified the more common lichen-gonidia with known free-living algae, yet the theory was received with a storm of opposition by nearly all lichenologists.
He investigated the exact relation of fungus and alga and showed that the same alga is able to combine with a number of different fungi to form lichens; thus Chroolepns umbrinus is found as the gonidia of 13 different lichen genera.
Moller also in 1887 succeeded in growing small lichen-thalli without their algal constituent (gonidia) on nutritive solutions; in the case of Calicium pycnidia were actually produced under these conditions.
The majority of the lichens, however, possess a stratified thallus in which the gonidia are found as a definite layer or layers embedded in a pseudoparenchymatous mass of fungal hyphae, i.e.
In the moist state of the thallus these colours are much less apparent, as the textures then become more or less translucent, and the thallus usually prevents the greenish colour of the gonidia (e.g.
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.
Funf stuck gives ten free living algae which have been identified as the gonidia of lichens.
They are distinguished by possessing as gonidia algae foreign to the ordinary part of the thallus.
The exact relation of gonidia and hyphae has been investigated e specially by Bornet and also by Hedlund, and very considerable differences have been shown to exist in different genera.
In Physma, Arnoldia, Phylliscum and other genera the gonidia are killed sooner or later by special hyphal branches, haustoria, which pierce the membrane of the algal cell, penetrate the protoplasm and absorb the contents (fig.
In many other cases, especially those algae possessing Pleurococcus as their gonidia, there are no penetrating hyphae, but merely From Strasburger's Lehrbuck der Botanik, by permission of Gustav Fischer.
In the first process, portions of thallus containing gonidia may be accidentally separated and so may start new plants.
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.
The perithecium is very constant in form and since the gonidia take no part variations are of value in classification some more details may be added.
In these forms gonidia are found in connexion with the young fruit; such algal cells undergo numerous divisions becoming very small in size and penetrating into the hymenium among the asci and paraphyses.
When the spores are thrown out some of these hymenial gonidia, as they are called, are carried with them.
When the spores germinate the germ-tubes surround the algal cells, which now increase in size and become the normal gonidia of the thallus.
The best-known species is Cora pavonia, which is found in tropical regions growing on the bare earth and on trees; the gonidia belong to the genus Chroococcus while the fungus belongs, apparently, to the Thelephoreae (see Fungi).
The genus Dictyonema has gonidia belonging to the bluegreen alga, Scytonema.
Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs.
Gonidia always belonging to the Cyanophyceae, Lichinaceae, Ephebaceae, Collemaceae, Pyrenopsidaceae.
This character becomes still more proas the gonidia of the lichenthallus.