;: THALLOPHYTES. 



this manner, the soredia can still further multiply outside the thallus ; but under favour- 



's oO^°oHc. 





Fig. ige^.—A—D soredia of Usnea barbata; A a simple sorediuni, consisting of a gonidium covered with a web of hyphre; 

 B a soredium, in which the gonidium has multiplied by division ; C a group of simple soredia, resulting from the penetration 

 of the hyphae between the gonidia : A E germinating soredia ; the hyphas are forming an apex of growth and the gonidia 

 are multiplying ; a-c soredia of Physcia parictma ; a with an envelope of pseudo-parenchyma ; 6 the envelope producing 

 rhizines; c a. young thallus formed from a soredium (after Schweiidener, X500). 



able conditions either a sin;?le soredi 



Fig. 196.— Various Lichen-gonidia ; A of Roccella 

 tiiictoria, i^-g"' in the act of multiplication, g g united 

 with branches of hyphee h; B of Everjiia di-varicata 

 united with a branched hypha ; C of Usnea barbata in 

 the act of division, at h united with a hypha ; D chain 

 of gonidia of Lichina pygmcea (after Schwendener). 



of growth of hyphcT and gonidia 



um or a mass of them grows out at once into a new 

 thallus (Fig. 195). Schwendener states that in 

 Usnea barbata this may occur while the soredia 

 are still included in the mother-thallus ; soredial 

 branches, as they are termed, are thus pro- 

 duced. 



We may now turn to the consideration 

 of the other elemental form out of which, in 

 addition to the Fungus-hyphae, the thallus of 

 Lichens is constructed, the Gonidia. It has al- 

 ready been shown that these are nothing but 

 Algae which are attacked and surrounded in 

 their growth by Ascomycetes, and serve as 

 hosts to them, the capability of assimilating in- 

 organic materials being wanting on the part of 

 their parasites. Every Lichen-forming Fungus 

 chooses a particular Alga, just as other parasites 

 like the Hypodermiae mostly infest only particular 

 hosts. The peculiarity of the parasitism of the 

 Lichen-fungi lies in the fact that they are not 

 attached to their host externally at any one par- 

 ticular spot, and do not penetrate into the cells 

 themselves, but become woven round them, and 

 thus enclose them in their hyphal tissue. Com- 

 plete unions of growth however sometimes take 

 place, single hyphae becoming closely attached 

 to the cell walls of particular Algae (or gonidia) 

 (Fig. 196, jrl g, Bg, Cg), a phenomenon which led 

 at one time to the assumption that the gonidia 

 are themselves products of the hyphae, the 

 branches of which swell up in places to a glo- 

 bular shape, and form chlorophyll. The opposite 

 view was also at one time held, that the 

 hyphae grow out of the gonidia (as, for in- 

 stance, by me in Bot. Zeitg. 1855, on Gollema). 

 But these very rare phenomena may now 

 be more easily explained as simple unions 



, and bv no means stand in the way of the 



