86 COMMUNAL LIFE OF ORGANISMS sect, ii 



to live apart from it. That the alga grows vigorously, multiplies rapidly, 

 and may even acquire larger cells than when free, may be nothing more 

 than an example of hypertrophy a pathological condition. It has been 

 suggested that the alga finds protection from desiccation within the fungal 

 mass ; but this seems to be scarcely necessary, as the algae in question 

 are certainly capable of enduring desiccation admirably ; moreover, it is 

 not the case that they secure real protection against desiccation, for 

 under given circumstances the lichen dries up so completely as to become 

 brittle. Besides this, the alga is prevented from multiplying in its most 

 efficient manner for instance, by zoospores. The alga is in a condition 

 of slavery in relation to the fungus, which is a kind of parasite differing 

 from ordinary parasites in incorporating the host and in providing a 

 portion of the food consumed in the host's maintenance. There is there- 

 fore a certain likeness to hemiparasitism, but we must assume that green 

 hemiparasites provide their own carbonaceous nutriment, whereas the 

 lichen-fungus needs merely to secure for itself the non-carbonaceous food- 

 material. 



In this case, too, the bond of union between the two organisms may 

 be exceedingly close, in that the fungus selects definite species of algae. 



MYCORHIZA AND ENDOPHYTES 



A mutualism characterized by complete reciprocity, in which the 

 symbiosis is equally advantageous to both partners, may or may not 

 occur. Even in the most familiar forms of symbiosis the relations sub- 

 sisting between the symbionts are not sufficiently understood to permit 

 of our completely explaining the nature of their connexion. This is 

 true of mycorhiza, in which the root of a highly organized plant enters 

 into intimate connexion with fungal hyphae, which are either ectotrophic, 

 and mainly form a sheath enveloping the distal surface, or are endotrophic, 

 and live inside the cortical cells. 



Mycorhiza has been found in the majority of Amentaceae, Coniferae, 

 Ericaceae, and many other plants, especially in perennial herbs growing 

 on humous soils, such as acid humus, peat, and mould. Mycorhiza is 

 present in the humicolous herbs, whether these do or do not contain 

 chlorophyll ; plants belonging to the latter category (saprophytes) seem 

 certainly to be dependent on the fungus for nutrition. The fungus 

 possibly derives some benefit from the phanerogam, and there is scarcely 

 a doubt that it is of use to the latter. 



As regards the endophytic mycorhizal fungus, Percy Groom ^ has 

 proved that in Thismia it indulges in an interchange of nutritive material 

 with the root, and promotes the production of protein bodies. The 

 fungus abstracts from the root certain substances and provides others 

 in exchange, and is itself perhaps digested to some extent. In the ecto- 

 trophic form the relations are different. 



Stahl has expressed the opinion that the mycorhizal fungus in all 

 cases undertakes the absorption of water and ash-constituents from 

 the soil ; accordingly, it would be of special importance in soils poor in 

 nutriment. This view harmonizes well with the distribution of mycorhiza. 



* Percy Groom, 1895 b. 



