I2O FIRST GROUP. THALLOPHYTES.' 



An interesting example of the formation of the thallus of a lichen is given in Stahl's 

 description of the behaviour of the hymenial gonidia. These are small gonidia 

 derived from the ordinary gonidia of the thallus, and found in the perithecia, 

 for instance, of Endocarpon pusillum in the intervals between the asci, and in the 

 jelly produced in the cavity of the perithecium by the swelling of the walls of emptied 

 asci. The gonidia of Endocarpon belong to the algal genus Pleurococcus. The 

 hymenial gonidia are specially and clearly distinguished from the gonidia of the 

 thallus by their minute size. The spores are set free from the perithecium simultane- 

 ously with the hymenial gonidia, and are closely encircled by them. The spores 

 if sown on glass or some other substance germinate at once and their germ-tubes 

 attach themselves to the gonidia. Striking changes are now perceived in the 

 gonidia ; they increase considerably in size and add largely to the amount 

 of chlorophyll which they contained both the result of the influence of the 

 Fungus. Other germ-tubes strike downwards into the soil and are the first 'rhizines.' 

 In the young thallus thus formed the separation into the various layers is only 

 gradually effected, the portion of the thallus which is above the ground being at 

 first only a mingled collection of gonidia and hyphae with scarcely any intervals 

 between them. If the hymenial gonidia vegetate apart from the hyphae, they remain 

 much smaller and multiply by divisions, the direction of which is different from that 

 of the gonidia of the thallus and agrees with that of the algal cells known as Sticho- 

 coccus ; the larger gonidia on the contrary, when vegetating in freedom, agree in 

 this respect with the gonidia of the thallus which are a form of Pleurococcus. The 

 hymenial gonidia of Endocarpon pusillum are employed in a still more remarkable 

 way by the spores of a small lichen, Thelidium minutulum, which occurs with Endo- 

 carpon^ in the formation of its thallus ; in this case we have an ascomycetous Lichen 

 constructing its thallus with gonidia obtained from another species. The part of the 

 thallus of Thelidium which has gonidia is of very reduced size, and forms a kind of 

 appendage only to the rest of the mycelium which runs through the substratum and 

 on which the perithecia are formed. In the earlier experiments of Rees, Treub and 

 Bornet, in which the spores of lichens were brought into connection with the Algae 

 which corresponded with their gonidia, the Fungus spun its threads round the Algae, 

 but no perfect thallus was formed like that produced in Stahl's experiment ; at the 

 same time the influence of the Ascomycete on the gonidia was very clearly shown. 



Formation of spores. The spores of Lichens are formed in fructifications, which 

 are known as apothecia, and which resemble the fructifications of the Discomycetes 

 or those of many Pyrenomycetes. The apothecia are formed inside the tissue of the 

 thallus and emerge from it at a later period, when they either spread out the flat 

 surface of their hymenial layer to the open air (gymnocarpous Lichens), or allow 

 their spores to escape by an orifice (angiocarpous Lichens). In all Lichens without 

 exception the apothecium with all its essential parts from first to last is produced 

 from the hyphal tissue only ; it is the Fungus alone which forms the fructification ; 

 the food-supplying Algae, the gonidia, either take no part in it or a very unimportant 

 part ; the tissue of the thallus with its gonidia merely grows up like a wall round 

 the apothecium and partly encloses it (Fig. 80), or grows more luxuriantly underneath 

 the apothecium and raises it as on a stalk above the surrounding thallus. The only 

 exception to the endogenous origin of the apothecium is to be found in Coenogonium 

 and similar forms, in which such a mode of formation is impossible, because the 

 hyphae form only a thin layer about the filamentous Alga that does duty for gonidia ; 

 these species plainly confirm what Schwendener's researches have demonstrated, that 

 the fructification in the Lichens belongs exclusively to the hyphal tissue. 



The apothecia of all the Lichens that have been sufficientiy investigated are the 

 result of a sexual act which agrees in many points with that of the Florideae. In 

 both cases there are spermatia which conjugate with a trichogyne, and a multicellular 

 conducting tube brings about the impregnation which causes the asci to grow out 



