Cultures of the moss, _Phascum cuspidatum_, were also made from the spores on a glucose medium. The specimens in association with the fungus were fully grown in two months, while the control cultures, without any admixture of the fungus, had not developed beyond the protonema stage. Servettaz draws attention to the proved fact that, in certain instances, plants benefit when provided with substances similar to their own decay products, and he considers that the fungus, in addition to its normal gaseous products, has elaborated such substances, as acid products, from the glucose medium to the great advantage of the moss plant.
A symbiotic association of _Nostoc_ with another alga, described by Wettstein[262], is also of interest. The blue-green cells were lodged in the pyriform outgrowths of the siphoneous alga, _Botrydium pyriforme_ Kütz., which the author of the paper places in a new genus, _Geosiphon_. The sheltering _Nostoc symbioticum_ fills all of the host left vacant by the plasma, and when the season of decay sets in, it forms resting spores which migrate into the rhizoids of the host, so that both plants regenerate together.
Wettstein has compared this symbiotic association with that of lichens, and finds the analogy all the more striking in that the membrane of his new alga had become chitinous, which he thinks may be due to organic nutrition.
Lichen hyphae form the ground tissue of the thallus apart from the gonidia or algal cells. They are septate branched filaments of single cell rows and are colourless or may be tinged by pigments or lichen acids to some shade of yellow, brown or black. They are of fungal nature, and are produced by the mature lichen spore.
The germination of the spore was probably first observed by Meyer[263]. His account of the actual process is somewhat vague, and he misinterpreted the subsequent development into thallus and fruit entirely for want of the necessary magnification; but that he did succeed in germinating the spores is unquestionable. He cultivated them on a smooth surface and they grew into a “dendritic formation”—a true hypothallus. Many years later the development of hyphae from lichen spores was observed by Holle[264] who saw and figured the process unmistakably in _Borrera_ (_Physcia_) _ciliaris_.