MUTUALISM. 87 



individual. Tliese, and other examples, will serve to illustrate 

 how we have in the lichen an organism with peculiarities of 

 structure and of life, widely differing from those of either an 

 alga or a fungus. This unification of two living beings into an 

 individual whole, I have designated " Individuation." ^ 



In the case of the lichen-symbiosis, the chlorophyllous part 

 consists of minute algal cells, completely enclosed in a tissue 

 of fungus-hyphae, and the lichen lives as a perfectly isolated 

 and independent plant. The case is, however, different where 

 the fungus enters into parasitic relationship with the green 

 cells of a large plant. Union may then take place, so that the 

 fungus lives on, or inside its host, and removed from contact with 

 any other substratum. The fungus is, however, not in a position 

 to convey any nourishment to its host, and in fact is absolutely 

 dependent on it for the organic substance and water necessary 

 for growth. Where, however, the relationship is such that the 

 parasitic fungus is still in contact with some other substratum, 

 then it may be assumed that, in spite of its parasitism, it 

 takes up nutriment from this source, and shares it with its 

 host. This, as has already been pointed out, is the state of 

 things in the lichens, where the fungus completely envelopes 

 the small isolated algae, and must, as a condition of the growth 

 of the lichen, remain in direct contact with the substratum ; 

 the fungus is believed to take from the substratum water and 

 inorganic food-material with which it supplies the algae, while 

 it receives in return plastic organic substance to be used in 

 its own growth. Of course cases do occur amongst the lichens, 

 where, in moist places, the alga is not dependent on the fungus, 

 or, on the other hand, where the fungus can itself take up 

 organic substance from its substratum. 



Another example of the case is the union of fungi with 

 non-chlorophyllous plants which inhabit humus (e.g. Monotrojm). 

 Here the fungus takes up organic nourishment from the 

 substratum and supplies it to the higher plant, which, in 

 consequence of its lack of chlorophyll, is directly dependent on 

 the plastic organized substance from the soil, supplied through 

 the agency of the fungus. The latter, however, receives nothing 

 in return ; it requires nothing, since its substratum offers it 

 the most favourable conditions for nutrition. This form of 

 ' Iiicli\-i(Uuili.siiuis. 



