512 The American Naturalist. [June, 
lichens, where the fungus forms an extensive thallus in a 
zone of which are contained the alge upon which it subsists, 
may be traced in existing species. Not only this, but there are 
genera, as has been said, in which there are species that do not 
attack alge, but live independently as saprophytes, and the 
point to be noticed here is that these genera belong to the 
intermediate group of what I may call, pseudo-heteromerous 
lichens. a cept pi 
These considerations, of course, do not prove the existence of 
mutualism in lichens, but they deprive it of much of its seem- 
ing unreasonableness. Other facts, now well established, 
make it certain that this relation really does exist in the 
heteromerous lichens. 
Arthonia is one of the pseudo-heteromerous lichens. More- 
over, it is one of those genera in which certain species, during 
their entire existence, live independently as saprophytes. Of 
its development, De Bary says: “ . . . . the hyphe of 
the thallus make their way into the outer layers of the peri- 
derm in the smooth stems of oaks and ashes and there grow as 
saprophytes independently, that is, without alge, into a thallus 
formed of an abundance of slender hyphe which spread 
through the cells of the periderm. Then its proper alga, 
Chroolepus umbrinum, finds its way from without through the 
cell walls of the peridermis into the previously formed hyphal 
thallus and is seized by it. The cells of the Chroolepus are in 
rows forming filaments with apical growth, and it is by means 
of this growth that they penetrate into the thallus in the same 
way as mycelial hyphe pierce through membranes. Thealga 
is a frequent inhabitant of the bark of trees, and makes its way 
into the periderm for its own purposes. Its penetration into 
the thallus of the fungus can scarcely be supposed to be caused 
by the fungus, but is merely an adaptation which favors the 
formation of a lichen.” This is plainly an ordinary case 0 
parasitism’ on the part of the lichen, but it not only throws 
light on the origin of the true heteromerous lichens, but it 
shows in what manner the fungus may be of benefit to the 
alge. In the heteromerous lichen the thallus takes the place 
of the bark of the tree in these pseudo-heteromerous lichens 
