MYCORRHIZAL SYMBIOSIS IN APLECTRUM, CORALLORHIZA AND PINUS

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Abstract
The symbiotic association of C. maculata with an endophytic fungus (Basidio-mycete) is characterized by profound morphogenic altera- tions including loss of roots, development of underground coralloid branches inhabited by the endophytic fungus which sends hyphae out through trichomes simulating roothairs. Foliar organs are represented by bracts on the flower scapes which contain only traces of chlorophyll. A. hyemale, a near relative of Corallorhiza, displays a tendency to muta-tive alterations similar to the above. Each annually formed crop of short lateral roots of Pinus may enter into symbiotic association with soil-fungi; sometimes not until the 2d season of the plantlet, but the implied symbiosis is necessary for the normal development of the tree. That the association is a symbiotic one is evidenced by the fact that isolated segments of branching roots, forming mycorhizae annually may carry on independent existences in the soil indefinitely; secondary tissues are not developed. The fungus forms a felted layer around the short roots as in ectotropic mycorhizae; in addition hyphae penetrate the cortex by traversing middle lamellae with occasional branches into the cytoplasm between vacuoles, an arrangement constituting ectendotropic mycorhizae. The hyphal branches are not placed with special reference to nuclei, which remain unmodified. This is in contrast with the endophytic mycorhizae of Aplectrum and Corallorhiza in which hyphal coils are formed near the abnormal nuclei. Penetration by the hyphae is limited by the action of phenol-oxidases which may present entrance into any cell. The phenol-oxidases and lipid-oxidases in the endodermis and pericycle form a regional barrier to the stele, rich in high energy P compounds, in itself an indication of nonparasitism. The 2d determining feature is the course of P metabolism. The fungus absorbs inorganic phosphates as well as C and N compounds from the soil, builds high energy P linkages, from which inorganic phosphates are released and translocated to irreversible group acceptors in the tissues of the roots. The origination of auxins and vitamins of the B group in hyphae and their translocation into roots were observed. Carbohydrates, never but sparingly present in fungi, were not found, but C compounds translocated to the cortex were synthesized into starch in abundance, even in isolated soil cultures. Sites of high oxidase activity in the boundary phases of vacuoles were identified and in regions of the meristem, cortex, endodermis, and pericycle. Vacuoles of one and the same cell might differ widely as to their colloidal contents. In addition, interphases of a 2d order were found in vacuoles of cells in the post meri-stematic stage in which globular aggregates of oxidized phenols were enclosed by phospholipidic alveolated membranes, forming coacervates. While the morphogenic characters of the mycorhizae of Aplectrum Corallorhiza, and Pinus differ widely, absorption and metabolization of soil substances and the processes of translocation and utilization of the nutritive material thus acquired are similar in the 3 cases. That the association is symbiotic and not parasitic is illustrated by the fact that isolated mycorhizal segments of pine roots may grow indefinitely like a chlorophylless plant.

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