WE have considered one particular aspect of the relation between the association of two organisms which is called disease, but other combinations can be found and a study of any throws light on them all. There are for example a number of yeasts associated with certain insects, and this association is not merely an external one like the fungus gardens cultivated by the termites of South America, but is actually internal. Pocket-like yeast-glands associated with the alimentary canal are found, and apparently the yeasts play an essential part in the digestion of cellulose for the insect. Presumably the yeast feels the benefit of a sheltered life and feeds on food that comes drifting by. It can be seen that the change from such a condition to a parasitic relation might be very slight.
This type of association of fungi with other organisms is
known as ‘symbiosis.’ Originally by ‘symbiosis ‘was understood an intimate association of two organisms for the benefit of each. It has, however, to be admitted that ‘mutual benefit ‘of two organisms is a matter more often of conjecture than of proof. Nevertheless association of fungi with other organisms close enough to be ‘symbiotic ‘is frequent.
FUNGI THAT NURSE THE ORCHID’S SEEDS
TWO interesting examples that may be quoted are provided by the association of certain kinds of microscopic fungi of the soil and, and by toadstools and the of many forest trees. The of are very small with an incomplete differentiation of parts and practically no arrangement for storing food. In some orchids, the majority of the die as, unless a germinating encounters the right kind of soil fungus, further development does not occur. Apparently the meeting of seed and fungus results in a stimulation of activity in the seed, which then develops a swollen structure about one-eighth of an inch in diameter, containing the fungus in the basal cells. The fungus starts by destroying some of the cell contents of the and then apparently the orchid asserts itself and prevents the fungus from destroying any further cells. The manner in which the fungus is restricted to a zone in which it can do no harm is indeed remarkable; it is kept far away from the point at which the young orchid’s shoot and develop. Eventually the young root absorbs its way right through the zone where the fungus is in occupation, but the root is protected as it goes, by a special layer, and eventually finds its way into the soil. Here, in the course of time, the fungus hypha? from the soil find their way into the outer series of cells of the root, and every orchid carries with it in this way the fungus that it requires for development of its seed.
This has become the basis of a spectacular application of biology to industry. An orchid grower contrived to take the fungus that he wanted from a number of cultivated orchids and grow them on nutrient jellies in the same way as an expert will cultivate bacteria. The fungus was then added to the orchid seed at the required moment, and so successful has this method become, that raising orchids from seed has been changed from a hazardous enterprise into a certainty. Of recent years, particularly in America, successful attempts
have been made to imitate the action of the fungus by supplying certain chemicals to the orchid seeds.
Practically all the woodland toadstools of this country are concerned in an association with the roots of our forest trees. Typically fungus mycelium is wrapped round the ends of young tree-roots that lie close to the surface. Eventually, however, the fungus penetrates the tissue of the root, but apparently is never permitted to become a nuisance. It is obvious that if the conducting area that lies in the middle of a root became blocked with masses of fungus hyphaj the whole plant would suffer, as even from mechanical interference alone the streams of water and food substances, flowing through the root, would be stopped. In tree-roots, as in orchids, the fungus is restricted to the outer cells and is never allowed to interfere with the food stream.
So far as any judgment can be reached on such a matter as this purely from a study of the distribution of fungus hyphaj in a host, such associations were common in a remote geological past, as fossils from the Coal Measures have frequently been found showing masses in their root-cells that have been interpreted as fossilised masses of fungi. A striking fact that does emerge from an examination of these associations is that the flowering plant always seems to be well in command of the situation.
What these associations mean and how they are to be interpreted is a difficult matter to decide upon, and to-day we are far from any final view on the matter.