1887.] MICROSCOPICAL JOURNAL. 25 
portant; also a sufficient, although small, supply of carbon dioxide. Finally, 
there must be exposure to sunlight ; but the intensity should be moderate, for 
it is known that too intense light checks growth. Witness the use of whitened 
glass in greenhouses. 
We are now prepared to contrast the conditions under which free terrestrial 
alge are found with what we can see must be the conditions in the interior 
of a lichen. In such a place as the surface of a rock, or the bark of a tree, 
the moisture is usually soon dried away, and the sunlight is often very intense. 
Under ordinary circumstances, there is no lack of all the carbon dioxide that 
is needed ; but possibly, at very high altitudes, the amount may be somewhat 
scanty. Conceive, now, a tiny alga to be within the hyphal net-work of a 
lichen. When it rains the felt-like mass of filaments eagerly absorbs the 
moisture and holds it like a sponge for a considerable length of time. The 
thin layer of cortical tissue above the alga is translucent, especially when wet, 
so that at just the time when sunlight is needed a proper amount is allowed 
to reach the little plant. It is well known that when fungi are growing 
they give out carbon dioxide in much the same way that an animal does in 
breathing. We may conclude, therefore, that the lichen-hyphe do likewise ; 
and thus we have a relation established with the alga not unlike what we find 
to exist between the animals and the green plants of a well-balanced aqua- 
rium —the one absorbing ‘oxygen and respiring carbon dioxide, the other 
assimilating the carbon dioxide, decomposing it, and returning the oxygen 
to the water. 
If we accept the Schwendener theory of lichen construction, as it is under- 
stood to-day, we have not only an explanation which harmonizes best with 
all that we know of plant life in general, and the life of lichens in particular, 
but we have presented for our study the phenomena of an association of two 
widely dissimilar kinds of plants, from the union of which mutual benefit is 
derived—that is to say, we have a vegetal symbtosts. 
Mapison, Wis., fanuary, 1887. 
Key to Genus Grynus. 
We insert the following convenient key to the species of the genus Grynus 
for the benefit of collectors of aquatic material who may chance to run 
against these beetles. The key is from the Journal of Microscopy, and ap- 
plies particularly to British species. 
Chart of spectes of the genus Grynus. 
Minutus, Fab. 
Punctures on elytra finer toward suture.— Urinxator, 
Ill. 
Punctures on elytra scarcely feebler toward suture.— 
Wnderside entirely rust-red) .20....00..0sccessss-scccesveccecas cecenace 
[ Body ovate or 
eval. Natator, Scop. 
Punctures on elytra distinctly finer toward suture,— 
Punctures scarcely finer.—Suffrzanz, Scrip. 
: Body elongate 
[ Reteged margin | 'obtang “with | 
++..+e.-—Bicolor, Payte. 
: nearly parallel @ 
elytra reddish. | Sides: ae 
Underside whol- 
ly or chiefly Interstices on elytra impunctate.—Déstinctus, Hub. 
black, legs Body oblong } Interstices indistinctly punctured.—Caspzus, Men. 
reddish. ovate. Interstices closely and distinctly punctured.— Colym- 
[ bus, Fr. 
Punctures on elytra scarcely finer toward suture.— 
Reflexed margin brassy black.......... Marinus, Gyll. 
Punctures much finer.— Ofacus, Sahlb. 
