542 THE AMERICAN NATURALIST. (VoL. XXXIII. 
‘uniformly distributed through the body of the thallus. This differ- 
ence has little systematic value. With few exceptions, some of 
which are still disputed, the lichen fungi all belong to the Ascomy- 
cetes. The lichen alge belong to the Schizophyceze and Chloro- 
phyceze. Rarely filamentous forms occur. The various lichen fungi 
have not been found growing free except in case of a few Basidio- 
lichenes, and some saprophyte Ascomycetes which occasionally form 
so-called “ Half-lichens’’; but in a number of cases they have been 
cultivated free. Many, if not all of the lichen alge, are also believed 
to occur free in damp places, but owing to changes in shape and 
size, brought about by the presence of the fungus, there is still much 
uncertainty respecting the identity of the algal element in the thallus 
of many lichens. Quite a number of these alga have been cultivated 
independently of the fungus, and ten species found in lichen thalli 
have been definitely identified as well-known species occurring in- 
dependently of fungi. These belong to as many different genera. 
Almost always each species of lichen is adapted to one species of 
alga, but sometimes accessory species of algæ are present in the 
thallus, forming in connection with the lichen-fungus growths called 
cephalodia. The fungus provides the imprisoned alga with water 
and inorganic foods. It loses nothing itself by this symbiosis and 
gains the ability to live on bare rocks and other most resistant sur- 
faces. What benefit the algæ derive from such unions is problematic. 
In the lichen thallus they are said to reach a larger size and to 
divide more rapidly than outside; they are also able to live and 
work in places otherwise unsuitable. On the other hand, they have 
very generally lost the ability to produce swarm spores, a function 
soon regained on cultivating them free from the fungus. The lichen 
attacks the rocks by means of acid excretions which the fungus alone 
appears to be unable to produce. More than sixty acids have been 
isolated and described, and the chemistry of the group is complex. 
The reproduction of the group, aside from mere vegetative propaga- ` 
tion by soredia, which is very common, is that of the fungus. Oidial, 
or chlamydospore, fructification has been found only in Caliceæ. 
Free conidial fructification, so common in the Ascomycetes, has been 
found only in two lichens, Pycnidia, on the other hand, are common. 
The highest form of fructification is by means of ascospores. In 
some lichens this form is wanting, or at least has never been found. 
Basidiospores are said to occur in the tropical genera Cora and 
Corella. According to the author it is exceedingly probable that 
even in Collema there is no sexual reproduction. Lichens are often 
