142 
Mycologia 
whether the hyphae enter the algal cells or not; but the injury to 
the host cells probably goes on more slowly in the latter case. 
Peirce (98, 99) has decided in favor of parasitism of the lichen 
upon the alga, but inconsistently holds that the lichen and its host 
somehow form a dual organism in spite of this antagonistic rela- 
tion. He found that while the algal host cells often multiply 
rapidly in lichen thalli, they increase yet more rapidly outside the 
lichens. During wet weather the alga grows more rapidly than 
the lichen ; but during dry weather the lichen grows more rapidly 
than the alga. In this manner the algal host cells are preserved 
from complete extermination by the lichen parasite, though some 
host cells are constantly being killed. In parasitism on multicel- 
lular organisms, portions of the hosts are likewise often killed 
while new portions are being produced by growth. The cells of 
the unicellular or filamentous lichen hosts separate as they are 
formed, or constitute short filaments. The isolated cells or the 
filaments are not so able to overcome the harmful effects of 
parasitism as are the aggregates of cells in multicellular hosts, in 
which food supply may pass from cell to cell so that the cells on 
which the parasite feeds may be nourished from surrounding cells. 
But the algal hosts of lichens are by no means the only ones that 
are killed by parasites ; nor are they the only ones in which the 
fungus kills only a portion of the host cells. Water, mineral salts, 
and probably some organic matter are carried to the imprisoned 
host. The parasite is not supposed to elaborate this material, 
more probably acting only as a carrier of food. Another pecul- 
iarity of the parasitism of lichens upon algae is that the algal 
hosts are usually surrounded completely by the parasite. i\Iore- 
over, the lichen is usually parasitic on a large number of individ- 
uals instead of a single one. 
Peirce says in his text-book of plant physiology (100) that 
“ because of the small size of the alga, the always larger fungus 
can not become entirely enclosed in it ; on the contrary, the fungus 
surrounds the alga with a more or less firm mycelium, containing 
the alga between the parts of its body. The association of the 
fungus and alga, always intimate enough for the fungus to supply 
itself osmotically with non-nitrogenous foods elaborated by the 
alga, is in many cases so exhausting to the alga that many of its 
