1893.] Symbiosis and Mutualism. 513 
The gonidia of the heteromerous lichens are usually Palmel- 
laceze, which, from their different structure and mode of growth, 
have not the power of getting beneath the bark as does Chroo- 
lepus. The thallus of the lichen serves the same purpose 
with them—protecting the colony of alge and absorbing 
and retaining unusual quantities of moisture, and enabling 
them to live and multiply in places where, under ordinary 
conditions, life would be impossible. 
That the thallus does do this is shown by the fact that 
lichens grow in places where alge could not maintain them- 
selves unaided, and by the fact that the gonidia multiply with 
great rapidity in the thallus, often more so we are told than 
without, and the individual cells attain a larger growth within 
the thallus than without, as has been shown by taking algae 
from the thallus and cultivating them independently. That 
the fungus does not do all this for nothing, the numerous 
exhausted cells to be found in the gonidial zone of any ordin- 
ary lichen abundantly testify. 
There is another curious phenomenon exhibited in some 
lichens. In these species the alge are not confined to the 
gonidial zone, but grow up into the tissues of the sporocarps 
between the paraphyses and among the asci, so that when the 
ascospores are ejected, cells of the alge are ejected with them 
and are promptly seized upon by the germinating spores. This 
can hardly be accidental, and it should be observed that it is 
the alga which is the moving party, not the fungus. Surely 
some benefit must result to the alga or it would not be done. 
It is possible, also, that there are other adaptations resulting 
from the symbiosis of fungus and alga in the lichen. Frank 
claims to have discovered several, one of which deserves men- 
tion. It is well known that alge can be separated from the 
lichen, and that they will then vegetate in the ordinary way 
independently. Frank asserts that certain species of alge 
ave become so adapted to life in the lichen and so accustomed 
to it, that they have partially or wholly lost the power of inde- 
pendent growth. No examples of this, however, are certainly 
known. 
