250 THE AMERICAN NATURALIST. [Vor. XXXIV. 
place between the hyphz and the invested gonidia. That such’ 
a movement does not take place it is impossible to believe unless 
one assume that the nature of the limiting membranes of algal 
and fungal cells is such that osmosis is a physical impossibility. 
This assumption would be self-destructive, however, for if these 
membranes were impervious to the dissolved substances in the 
algal and fungal cells, all osmosis would be impossible, and the 
cells would all die from lack of food and water. There must 
then be osmotic movement between fungus and alga. In 
obedience to the ordinary laws of osmosis there would be move- 
ment of dissolved substances from regions of more to regions 
containing fewer molecules of. these substances. The alga 
under the influence of light manufactures complex nutritious 
non-nitrogenous carbon compounds, which are at times, if not 
always, in soluble forms. These substances would tend to 
migrate from the algal to the fungal cells. This physical 
phenomenon is of the utmost physiological importance to the 
fungus, for it thereby gains the food it needs. Such a transfer 
is inevitable so long as the conditions of intimate contact, of 
permeable membranes, of supply of food and demand for it con- 
tinue the same. If the demand for food by the fungus or any 
part of it exceed the ability of the alga, or of any algal cell, to 
manufacture enough for its own needs and those of the fungus, 
the fungus will consume the alga itself. The numbers of empty 
gonidial cells in the lichen body are sufficient evidence of this, 
but this evidence cannot always be found, for only at times 
does the fungus demand so much of the alga that the body 
substance of the alga must be given to feed the fungus. 
The slow-growing fungus component of the lichen draws 
food from the more rapidly growing algal cells in which the 
food is manufactured. Such taking of food is evidently para- 
sitism. It can be nothing else. The intimacy of contact of 
hyphz and gonidia precludes any other supposition than that 
the hyphz osmotically draw food from the necessary and help- 
less alge. If the alge grew and multiplied less rapidly, or if 
the fungus grew more rapidly, or if the alga made less and the 
fungus demanded more food, or even if fungus and alga always 
1 Peirce, G. J. Loc. cit., p. 225, etc. 
