Mycorrhiza

Many plant diseases are caused by fungi, too many, in fact, to catalog here. Silkworm disease, brown rot of plums, early blight of potatoes, Fusarium wilt of cotton, a smut disease of sugar cane are but a few. Certain other fungi, however, are worthy of praise.

Fungi assist in the germination of seeds. The mycelia of various basidiomycetes and ascomycetes are associated with the roots of flowering plants in symbiotic relationships. The name for such an association is mycorrhiza (myco meaning “fungus,” rhiza meaning “root”). The fungi involved in such relationships are present in the soil. The fungal hyphae connect to the younger portions of roots and form a sheath of compact mycelium around the roots. Pines, heaths, and orchids lack root hairs and can grow successfully only when they associate in this way with fungal hyphae that furnish the absorbing mechanism. When pines are planted in an area where they have not grown before, the forester mixes the proper fungi with the soil to encourage successful growth.

Two kinds of union are recognized: an ectotrophic association, wherein the hyphae of the fungus grow between the cortical cells of the root but do not, or only rarely, penetrate the root cells; and an endotrophic association characterized by the absence of the fungal sheath and wherein the hyphae penetrate the cortex of the root and enter the cells. In endotrophic mycorrhizae, the hyphae that penetrate the cells disintegrate over time and are then digested by the host cells. This liberates certain products that can be absorbed by the roots. Perhaps one hundred species of fungi are capable of forming mycorrhizae with forest trees.

The mycorrhizal fungi may have special nutritional requirements, such as carbohydrates and vitamins, and it is presumed that they derive these benefits from tree roots. The reverse condition also occurs. A conifer grown in conditions wherein it is denied association with fungal hyphae often fails to develop normally but can sometimes recover when inoculated with a suitable fungus.

The mycorrhiza of orchids is endotrophic, the mycelium being widely distributed through the cortex or, if the orchid is a rootless form, penetrating the chlorophyll-free tissues of absorbing organs. The minute seeds of orchids do not germinate unless they are infected with the appropriate fungus, and until they have emerged aboveground, they are totally dependent on the fungal infection.

Consider the following scenario. A crayfish living in the darkness of ac ave loses the capacity to form eyes. A fungus that ensnares worms cannot produce the mechanism of entrapment in the absence of the worms. A tree having mycorrhiza loses the capacity to produce root hairs. In chapter 5 on Mendelian genetics, some discussion centered on Lamarcks concept of the inheritance of acquired characteristics, wherein organisms achieve traits to fulfill needs. Would the preceding examples, then, illustrate the disinheritance of unneeded characteristics? It sounds like reverse Lamarckism. What, then, causes the disappearance of genes for unused traits?

Insects are involved in some instances. For example, certain scale insects may press closely against the smooth bark of a tree, where they both suck the juices of the plant and become infected with a fungus. The fungus may derive benefits from the plant indirectly by absorbing plant nutrients from the insects.