Careful cultures were made of different Cladonia gonidia which were morphologically indistinguishable, and which varied in size from 10 to 16µ in diameter, though smaller ones were always present. He recognized them to be species of Cystococcus: they have a pyrenoid[257] in the centre and a disc-like chromatophore more or less starred at the edge. These gonidia grew well on agar, still better on agar-glucose, but best of all with an addition of peptone to the culture. There was invariably at first a slight difference in form and colour in the mass between the gonidia of one species and those of another, but as growth continued they became alike.
In testing for carbon supply, he found that gonidia grew slowly without sugar (glucose), and that, as sources of carbon, organic acids could not entirely replace glucose though, in the dark, the gonidia used them to some extent; the colony supplied with potassium nitrate, and grown in the dark, had reached a diameter of only 2 mm. in three months. With glucose, it measured 5 mm. in three weeks, while in three months it formed large culture patches.
A further experiment was made to test their absorption of peptones by artificial cultures carried out both in the light and the dark. The gonidia grew poorly in all combinations of organic nitrogen compounds. When combined with glucose, growth was at once more vigorous though only half as much in the dark as in the light, the difference in this respect being especially noticeable in the gonidia from Cladonia pyxidata. He concludes that as gonidia in these cultures are saprophytic, so in the lichen thallus also they are probably more or less saprophytic, obtaining not only their nitrogen in organic form but also, when possible, their carbon material as glucose or galactose from the hyphal symbiont which in turn is saprophytic on humus, etc.
B. Nutrition of Fungi
Fungi being without chlorophyll are always indebted to other organisms for their supply of carbohydrates. There has never therefore been any question as to the advantage accruing to the hyphal constituent in the composite thallus. The gonidia, as various workers have proved, have also a marked preference for organized nourishment, and, in addition, they obtain carbon by photosynthesis. Chodat[258] considers that probably they are thus able to assimilate carbon-dioxide in excess, a distinct advantage to the hyphae. In some instances the living gonidium is invaded and the contents 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[259], 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[260]. In his study of mosses, he was able to confirm Bonnier’s[261] account of lichen hyphae growing over such plants as Vaucheria 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[262], is also of interest. The blue-green cells were lodged in the pyriform outgrowths of the siphoneous alga, Botrydium pyriforme Kütz., 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.