Symbiotic association with other animals, in varying degrees of interdependence, is frequent.
Weiss has obtained interesting evidence that the symbiotic association between roots and Fungi, known as " Mycorhiza," already occurred among Carboniferous plants.
This is especially the case in the lichens (symbiotic organisms composed of a fungal mycelium in association with algal cells), which are usually exposed to very severe fluctuations in external conditions.
Fixation of Nitrogen.Another, and perhaps an even more important, instance of symbiotic association has come to the front during the same period, it is an alliance between the plants of the Natural Order Leguminosae and certain bacterium-like forms which find a home within the tissues of their roots.
Very complete examination, however, has now been made of many instances, and the name mycorhila has been given to the symbiotic union.
Size.) der Botanik, by ascolichens the fungus portion seems to have become so specialized to its symbiotic mode of life that it is never found growing independently.
It is very probable that numerous symbiotic fermentations in the soil are due to this co-operation of oxygen-protecting species with anaerobic ones, e.g.
2.-I, Portion of epithelium from the tentacle of an Actinian, showing three supporting cells and one sense cell (sc); 2, a cnidoblast with enclosed nematocyst from the same specimen; 3 and 4, two forms of gland cell from the stomodaeum; 5a, 5b, epithelio-muscular cells from the tentacle in different states of contraction; 5c, an epithelio-muscular cell from the endoderm, containing a symbiotic zooxanthella; 6, a ganglion cell from the ectoderm of the peristome.
Mycorhizas.The most interesting cases, however, in which Fungi form symbiotic relationships with green plants have been discovered in connection with forest trees.
These are: (I) That the green plant is so stimulated by the symbiotic association which leads to the hypertrophy, that it is able to fix the nitrogen or cause it to enter into combination.
It is, in fact, fully established that these leguminous crops acquire a considerable amount of nitrogen by the fixation of the free nitrogen of the atmosphere under the influence of the symbiotic growth of their root-nodule-microbes and the higher plant.
The lichen algae are not alone in their specializa tion to the symbiotic (or parasitic) mode of life, for, as stated earlier, the fungus appear in the majority of cases to have completely lost the power of independent development since with very rare exceptions they are not found alone.