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LICHEN GONIDIA 43
extract carbohydrates from many sources, so algae can secure their carbon
supply in a variety of ways. He affirms that the metabolic activity of the
alga in these cultural conditions is entirely normal, and the various cellcontents
are formed as in the light. Whether, in this case, starch is formed
directly from the acids or through a series of combinations has not been
determined. Uhlir 1
, with electric lighting, made successful cultures of
Nostoc isolated from Collemaceae on silicic acid, proving thereby that these
gonidia do not require a rich nutriment. A certain definite humidity was
however essential, and bacteria were never eliminated as they are associated
with the gelatinous membranes of Nostocaceae.
e. NUTRITION WITHIN THE SYMBIOTIC PLANT. Culture experiments
bearing more directly on the nutrition of lichens as a whole were carried
out by F. Tobler 2 . He proved that the gonidia had undoubtedly drawn on
the calcium oxalate secreted by the hyphae for their supply of carbon. In
a culture medium of poplar-bark gelatine he grew hyphae of Xantkoria
parietina, and noted an abundant deposit of oxalate crystals on their cell-
walls. A piece of the lichen thallus including both symbionts and grown on
a similar medium formed no crystals, and microscopic examination showed
that crystals were likewise absent from the hyphae of the thallus that had
grown normally on the tree, the inference being that the gonidia used them up
as quickly as they were deposited. It must be remembered in this connection,
however, that Zopf 3 has stated that where lichen acids are freely formed
as, for instance, in Xanthoria parietina, there is always less formation and
deposit of calcium oxalate crystals, which may partly account for their
absence in the normal thallus so rich in parietin.
Tobler next introduced lichen gonidia into a culture medium in which
the isolated hyphal constituent of a thallus had been previously cultivated,
and placed the culture in the dark. In these circumstances he found that
the gonidia were able to thrive but formed no colour: they were obtaining
their carbohydrates, he decided, not from photosynthesis, but from the
excretory products such as calcium oxalate that had been deposited in the
culture medium by the lichen hyphae. We may conclude with more or less
certainty that the loss of carbohydrates, due to the partial deprivation of
light and air suffered by the alga owing to its position in the lichen thallus,
is more than compensated by a physiological symbiosis with the fungus 4 .
It has indeed been proved that in the absence of free carbon-dioxide, algae
may utilize the half-bound CO2 of carbonates, chiefly those of calcium and
dissolved in water.
magnesium,
/ AFFINITIES OF LICHEN GONIDIA. Chodat 5
has, in recent years,
made cultures of lichen gonidia with a view to discovering their relation to
1 Uhlir 1915.
2 Tobler 1911.
3 Zopf 1907.
* Chambers 1912.
5 Chodat 1913