LECTURE VII 



MYCOTHALLI AND MYCORRHIZOMES 



General Character: — Mycothalli and mycorrhizomes are ordi- 

 nary liverwort gametophytes, fern stems and orchid rhizomes that 

 possess endophytes. Most of these structures in nature appear to be 

 invaded with fungi, for apparently most thalli and prothalli that are 

 not actually in water are mycotrophic, and most rhizomes likewise. 

 Here again the fortuitous character of the symbiosis is seen, since 

 apparently the fungi simply grow into these structures as into a part 

 of the environment ; and there is nothing evidently obligate about the 

 relationship. 



Mycothalli in Liverworts: — Their structure is detailed for 

 Pellia by Ridler (1922): In Pellia no plants were found entirely 

 without infection and usually the endophyte occurs in a definite zone 

 along the thickened median portion towards the ventral surface of the 

 thallus and in the rhizoids. Infection from the soil is presumably 

 through the rhizoids. Within the thallus, penetration of the cell-walls 

 seems effected mechanically ; the hyphae are swollen where their 

 growth is arrested by cell-walls, and they are constricted by passage 

 through them. The liverwort seems to exert some control over the 

 fungus and limits its invasion as stated, to a definite region in the 

 thallus. Here the hyphae form arbuscles or bushy-branched struc- 

 tures which later degenerate into sporangioles or little rounded bodies 

 that are insoluble in usual reagents. Formation of arbuscles stops 

 further growth of the fungus and this phenomenon caused Bernard 

 (1909) tO'term it an "immunity humorale". The effect of the fungus 

 in Pellia is very marked for protoplasmic content of invaded cells of 

 the thallus is killed, chloroplasts disappear and cells ultimately become 

 brown in colour. Starch disappears from cells of the thallus on 

 entrance of the fungus and is replaced by oil. When the sporophyte 

 is invaded (the thallus is of course the gametophyte) the contents 

 of the cells are wholly or partially absorbed. The fungus invades the 

 region of the sexual organs but does not grow into them.* 



♦According to Peyronel, the Jungermanniaceae are infected only by 

 mycomycetes. On poor soil, infestation dwindles with decrease of light (Nuovo 

 Gior.Bot.Ital. 49:362-382, 1942). 



