146 
]\Iycologia 
rapidly in free nature than in lichen thalli, where it shows a sickly, 
unnatural appearance, seen in the paler color, the less refractive 
condition of cell contents, and the common absence of the pyren- 
oids, all of which indicate parasitism of the lichen upon the im- 
prisoned alga. The injurious effects of the lichen upon the algal 
host are greatly lessened, he finds, when the lichen and its host 
are grown together on a favorable nutrient medium, where the 
lichen can secure niore of its nourishment from the substratum, 
and so depend less upon the host. Treboux’s conclusions, though 
based upon rather limited investigation, seem reasonable, are in 
accord with other recent results and demand a modification of 
some views commonly held concerning the relation of the lichen 
to its algal host. For instance, the lichen may at any time readily 
lay hold of free algae, an entirely reasonable supposition, which 
deals a death blow to certain untenable suppositions regarding the 
nature of lichens. According to his view also the food relation 
of the lichen to its algal host is not a very close one ; but his re- 
sults at this point are not based on careful investigation, and we 
are still disposed to believe that higher lichens depend largely upon 
their hosts for food. Whether Cystococcus humic ola is distinct 
from Chlorococcum is a problem very difficult of solution, and 
judgment may well be reserved, especially as this question scarcely 
affects any important matters concerning the relation of the lichen 
to its algal host. 
C. E. Bessey (22) says in “The Essentials of Botany,” “The 
plant body of a lichen is composed of jointed, branching, colorless 
filaments similar to those in the other families of this order, but 
more or less compacted together into a thallus or branching stem. 
They obtain their nourishment' from little green protophytes or 
phycophytes to which the filaments attach themselves parasitically. 
These little hosts, which live in the midst of the moist tissues of 
the lichens, were until recently supposed to be parts of the lichen 
itself. . . . There is thus an association between these plants which 
is mutually beneficial (symbiosis). The lichen lives parasitically 
upon the green plants, to which it in turn furnishes shelter and 
moisture.” Clements in his text (36) defines thus: “The type 
of parasitism in which the presence of the parasite benefits the 
host plants in some measure is commonly distinguished as sym- 
