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LICHEN GONIDIA 45
used up by the fungus and any dead gonidia are likewise utilized for food
supply. It is also taken for granted that the fungus takes advantage of the
presence of humus whether in the substratum or in aerial dust. In such
slow growing organisms, there is not any large demand for nourishment on
the part of the hyphae: for many lichens it seems to be mere subsistence
with a minimum of growth from year to year.
C. SYMBIOSIS OF OTHER PLANTS
The conception of an advantageous symbiosis of fungi with other plants
has become familiar to us in Orchids and in the mycorhizal formation on
the roots of trees, shrubs, etc. Fungal hyphae are also frequent inhabitants
of the rhizoids of hepatics though, according to Gargeaune 1
,
the benefit to
the hepatic host-plant is doubtful.
An association of fungus and green plant of great interest and bearing
directly on the question of mutual advantage has been described by
Servettaz 2 . In his study of mosses, he was able to confirm Bonnier's 3
account of lichen hyphae growing over such plants as Vaticheria and
the protonema of mosses, which is undoubtedly hurtful; but he also found
an association of a moss with one of the lower fungi, Streptothrix or
Oospora, which was distinctly advantageous. In separate
cultivation the
fungus developed compact masses and grew well in peptone agar broth.
Cultures of the moss, Phascum cuspidatum, were also made from the
spores on a glucose medium. The specimens in association with the fungus
were fully grown in two months, while the control cultures, without any
admixture of the fungus, had not developed beyond the protonema stage.
Servettaz draws attention to the proved fact that, in certain instances,
plants benefit when provided with substances similar to their own decay
products, and he considers that the fungus, in addition to its normal gaseous
products, has elaborated such substances, as acid products, from the glucose
medium to the great advantage of the moss plant.
A symbiotic association of Nostoc with another alga, described by
Wettstein 4
, is also of interest. The blue-green cells were lodged in the
pyriform outgrowths of the siphoneous alga, Botrydium pyriforme Kiitz.,
which the author of the paper places in a new genus, Geosiphon. The
sheltering Nostoc symbioticum fills all of the host left vacant by the plasma,
and when the season of decay sets in, it forms resting spores which migrate
into the rhizoids of the host, so that both plants regenerate together.
Wettstein has compared this symbiotic association with that of lichens,
and finds the analogy all the more striking in that the membrane of his new
alga had become chitinous, which he thinks may be due to organic nutrition.
1
Gargeaune 1911.
2 Servettaz 1913.
3 See p. 65.
4 Wettstein 1915.