STUDIES ON THE ECOLOGICAL AND PHYSIOLOGICAL SIGNIFICANCE OF AMPHICARPY IN GYMNARRHENA MICRANTHA (COMPOSITAE)

The 2 types of fruit (aerial and subterranean) borne by the dwarf desert annual Gymnarrhena micrantha were compared with regard to their responses to factors affecting their formation, dispersal, germination and seedling mortality. The 2 types of fruit differed markedly in several respects. In comparison with the subterranean fruits, the aerial ones are much smaller and more numerous, but the formation of the inflorescence in which they develop is more dependent on a favorable supply of soil moisture. The aerial fruits are dispersed by wind, after becoming detached by a complex series of hygroscopic movements which involve several organs and tissues, while the subterranean fruits never leave the dead parent plant, germinating right through its tissues. Germination of the subterranean fruits starts after a shorter incubation period and is less temperature-dependent in both light and dark. Light stimulated germination of both types of fruit, increasing their germination rates and final percentages, but not affecting the duration of the incubation period. In the subterranean fruits, the rate of germination was equally stimulated by light over the entire temperature range, with a well-defined optimum at 15 C in both light and dark. In the aerial fruits, the same optimum was found only in the light, rates in darkness increasing with decreasing temperatures. In the aerial fruits, alternations of light and dark were more favorable to germination than either continuous light or dark, the full effect being obtained with a single 8-hr or 16-hr light period, provided it was preceded by 16 or 8 hr of darkness, respectively. Similar reactions to combinations of light and dark were not observed in the subterranean fruits. Seedlings developing from the subterranean fruits were much larger, but grew at a relatively much slower rate than those from aerial fruits. The former were distinctly more tolerant of unfavorable soil-moisture regimes, such as low moisture supply and drought. It was concluded that the 2 types of fruit serve 2 distinct functions in the biology of the plant. The aerial fruits are adapted to the function of increasing the distribution of the species within suitable habitats, while the subterranean fruits are adapted to increasing the probability of the survival of the species.