Many other cases of symbiosis have been investigated with some completeness, especially those in which lower plants than the Phanerogams are concerned.
Numerous other cases of symbiosis have been discovered among the fungi of fermentation, of which those between Aspergillus and yeast in sake manufacture, and between yeasts and bacteria in kephir and in the ginger-beer plant are best worked out.
It now is - whether the free nitrogen of the atmosphere is brought into combination under the influence of micro-organisms, or other low forms, either within the soil or in symbiosis with a higher plant, thus serving indirectly as a source of nitrogen to plants of a higher order.
The discovery of the widespread occurrence of this mycorhizal symbiosis must be neld to be one of the most important results of research upon the nutritive processes of plants during the closing decade of the 19th century.
The union taking place underground, while the bulk of both partners in the symbiosis rises into the air, renders the association a little difficult to see, but there is no doubt that the plants in question do afford each other assistance, forming, as it were, a kind of partnership. The most pronounced case of parasitism, that of Cuscuta, the dodder, which infests particularly clover fields, appears to differ only in degree from those mentioned, for the plant, bare of leaves as it is yet contair.s a little chlorophyll.
The importance of the symbiosis can only be understood by considering the relationship in which plants stand with regard to the free nitrogen of the air.
This interesting case of symbiosis is equalled by yet another case.
For 50o years Austria had fulfilled this double task fairly adequately; but in its third task, that of turning a mechanical combination into an intimate union, a symbiosis of the nationalities, the State failed.
It must be borne in mind that the exact nutritive relations of the two constituents of the lichen have not been completely elucidated, and that it is very difficult to draw the line between symbiosis and parasitism.
The work of numerous observers has shown that the free nitrogen of the atmosphere is brought into combination in the soil in the nodules filled with bacteria on the roots of Leguminosae, and since these nodules are the morphological expression of a symbiosis between the higher plant and the bacteria, there is evidently here a case similar to the last.