DO PLANTS HELP ONE ANOTHER?

Otago Daily Times, Issue 19198, 14 June 1924, Page 3

DO PLANTS HELP ONE ANOTHER?

Written for the Otago D ally Times. By Fawcett Clappertox, B.Sc. There is an almost universal idea that in wild nature—nature red iu tooth and claw—competition is (ho general rule, with such exceptions as the care of animals for their males and for (heir young. But a more careful study reveals the fad that even among plants (here are many cases of cooperation between the most divergent forms, each bringing into (be common stock somethign which its partner cannot supply and receiving in return something in which itself is deficient. Tho fact, however. I hat these things were hidden so long only proves how severe is the contest that generally prevails between individuals and species for their footing upon the earth; and the exceptions to the rule .and the causes of their existence become all the more interesting. For ver,y many years the difficulty of germinating seeds of orchids has been well known to gardeners. In ordinary soil some species never begin to germinate at all; others pass through only a few of the early stages of growth and then die. Possibly through observing the success of selfsown seedlings growing among tho same loam us rhe parent plant it became the custom to sow seeds among soil (o which some of this loam had been added. The result was that practically every seedling developed, and it has become the common practice to propagate orchids in this way. But what was (he virtue in the loam lying about the roots of the parent plant? It wag known that the roots themselves were infccled by fungi, and putting two and two together it was conjectured that the growth of a young plant might be impossible without (he presence of a fungus. At first sight this idea is startling, for we are accustomed to think- of fungi as Ihe moulds, rusts, smuts, and scabs that make the gardener’s life anything but -a bed of roses. The next step was to try to isolate thi* fungus from the roots of tho orchid and this was successfully done, a pure culture being formed. Orchid seed was then sown and infected with the culture. Complete success resulted, and now you may read in an orchid grower's advertisement that his plants are raised by the "‘method of pure culture.” Now. what is the significance of this complete dependence of tho orchid seedling upon the fungus? It is impossible to say with certainty. There is apparently everything in the seed necessary for its growth, hut as an electric hell with everything in order and batteries charged does not ring till the button is pressed, so the seedling orchid is unable to grow without the stimulus provided bv the fungus—a stimulus which is probably chemical in its nature. Later on the roofs of the orchid become infected and retain the fungus either in the relation of host to" parasite or in a. symbiosis till the end of its life. The fungus gets, in either ca.se, from the orchid those organic foods, sugar or starch, which—being iteslf without green chlorophyll—it cannot make for itself. The association of fungal species with the roots of numerous flowering plants has long been known, but just what the balance of benefits is on either side is still much a matter for conjecture. Probably enough there are to be found all stages between that at which Ihe fungus is a predatory enemy of the higher plant and tho stage when, to speak figuratively, they have compromised with-one another, finding (hut a -state of warfare in which each flourished in proportion as the other decayed was far less profitable than a pooling of resources for, mutual benefit. Society has still something to learn from “the ITlies of the field”; it mav yet find that even among the plants co-operation is the, top notch of evolution. Although the instances of co-operation to a greater or lesser degree in nature are numerous, few’ have been so carefully studied as that of the common heath or ling. It presents some particular points of interest. Unlike those of the orchid, the seeds of ling can bo germinated "upon a medium which is quite sterile as far as the fungus peculiar to the parents’ roots is concerned. Yet when the seedling reaches the adult state it is found always to be infected. \Vhere does the fungus come from ? Obviously tho seed itself must have contained it; and that conclusion carries with it the further one that the fungus found in (he roots of ling must penetrate up the stem and into the flowers. Microscopic examination ofe the seed proved that- tho fungus was in the seed coat, and when (ho seed coat was sterilised it was found impossible to get the embryo to develop ; no fungus, no ling. The association of tho two plants is advanced several steps further than that of (he orchid and its fungus. Heaths grow like orchids, among soil rich in humus, but deficient in nitrogen. In a recent article it was shown how insectivorous plants hud solved tho problem of nitrogen deficiency. The ling is believed to have found a "better way when it formed a partnership with a fungus, for the latter, like the Bacterium radio icola found in the roots of legumes, is probably able to fix the nitrogen of the air and provide enough for the needs of its host as well as its own. The fungus first came as a foe. hut remained a guest —more, became one of, the family. The most remarkable instance of symbiosis (or commensalism, the sharing of a common table) is the lowly lichen that spreads over rocks and boulders, beautifying them with its quaint shapes and curious tints of orange, yellow, or grey. It may flourish for decades or even for centuries with no more sustenance than it can get out of rhe solid rock and from the atmosphere. it cun live for months through tho severest drought and endure cold or scorching heat with indifference. Everything on the surface points to its being a fungus ; yel it lives quite out of touch with those organic materials which a fungus must have or die. Only green plants are endowed with the power of using the energy of sunlight to make sugar and starch; and fungi, which are not green, must get these from some plant that has thorn. Wc may often see on damp walls or on the trunks of trees a green scum which is in reality a host of unicellular plants each endowed with the independence which comes of tho possession of chlorophyll. They are able to live and multiply indefinitely so long as there is enough moisture, but they are quite unable to endure drought like the. lichen. And here the'strange part of the story come? in, for il has been found that licheri is not a fungus only, but that it accommodates within its tissues a number of green plants like to those just describeaj The two live together in the closest association—the green alga protected from desiccation on every side by tho stout walls of the fungus and “expected” in return for this safe retreat; to provide enough sugar for the entire menage. The proof that this is the correct interpretation of what the microscope reveals is provided by the. synthesis of a lichen from a fungus and an 'alga. Spores of the fungus are sown among the cells of the alga. As they germinate (hey sent out their threadlike hvphac among the green colls anil fasten their suckers upon them. Nourished by what they get out of the algal cells they continue' to multiply and enclose them mil'll the whole mass, fungus- and alga, is a closely-interwoven tissue forming the beginnings' of a lichen plant.