6 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
use this literature, and it is therefore requisite to give some explana- 
tion of its leading conceptions and of the terms used. 
There are in all lichens, at least at some period of their life, certain 
cells or chains of cells of a green color, or more rarely of a blue-green, 
brownish, or reddish color. If green, these cells were called ‘‘gonidia” 
(fig. 1,a,p.9). If blue-green, they were called ‘‘gonimia” (fig. 1, ¢). 
Besides these cells or cell groups or filaments there are the well-known 
hyphe of the lichen thallus, which have in the phylogenetic develop- 
ment of lichens very frequently become transformed in part into a 
parenchymatous structure, the cortex. In extreme instances, the 
whole hyphal structure has been transformed into cellular structure, 
while, on the other hand, in many of the lower lichens the cellular 
structure is entirely absent and the green or blue-green cells and the 
hyphe compose the whole vegetative portion of the lichen. It was 
until recently supposed that the green or blue-green cells arose in the 
development of each lichen, probably from the hyphae, and that the 
relation of the chlorophyllous to the hyphal portion of the thallus was 
the same as that of the chloroplasts of higher plants to other portions 
of the plant body. Thus both the green or blue-green cells and the 
hyphe or equivalent parenchyma were regarded as integral parts of a 
single organism. It had long been known, however, that the green or 
blue-green cells in the lichen thallus are like certain algw (figs. 1, 2, 
pp. 9, 10), and De Bary, the first botanist to investigate the resem- 
blance carefully, advanced the suggestion that the alga-like cells of 
lichens might actually be alge somewhat modified by peculiar con- 
ditions of existence. Schwendener, at first hostile to this suggestion, 
in 1868 announced his conviction, based upon the examination of 
lichen thalli, that the lichen is composed of two distinct portions, a 
fungal and an algal. Later investigations have established beyond 
doubt the main points of this view and have shown that the alge 
in many lichens may be isolated, in which case they behave very 
much like similar free alge. Some investigations of the fungal por- 
tions of lichens followed, and the conclusion was reached that the 
lichen could not be regarded as an autonomy at all, but must rather 
be considered a compound organism composed of a fungus and an 
alga, the two living together in the relation known as symbiosis. 
Symbiosis, however, is of different kinds. It may exist with benefit 
to both organisms, the relation then constituting mutualism or 
mutualistic symbiosis; or it may exist with benefit to one of the sym- 
bionts and injury to the other, constituting parasitism or antagonistic 
symbiosis. In the early days of the investigation of symbiosis in 
lichens it was supposed that the fungus member which produced the 
fruit was benefited by the association while the alga was injured, the 
fungus thus being regarded as a parasite and on the alga as its host. 
