Spore Germination and Viability in Pteridophyta: Evolutionary Significance of Chlorophyllous Spores

Abstract

Chlorophyll-bearing spores occur in only a few unrelated families and genera in the Pteridophyta, including Equisetaceae, Osmundaceae, Grammitidaceae, Hymenophyllaceae, Blechnum (B. nudum), Lomariopsis sorbifolia, and all species of Christiopteris, Marginariopsis, Matteuccia, Onoclea, and Onocleopsis. These spores germinate in less than three days (mean=1.46 days) and have viability lengths of one year or less (mean=48 days). Non-chlorophyll-bearing spores occur in the majority of ferns, require longer periods for germination (4-210 days; mean=9.5 days), and have longer viabilities (mean=1045 days). Gametophytes from green spores also appear to develop more rapidly than those from non-green spores. It is suggested that rapid germination and relatively short viability of green spores is due to their constant respiration and apparent lack of dormancy. Species with green spores occur consistently in wet mesophytic habitats, and it is suggested that lack of selective pressure for dormancy has maintained this condition in Equisetaceae and Osmundaceae. In the other groups of ferns, the condition is most likely derived. BASIC TO THE reproduction of any individual fern is the ability of its spores to germinate and for the resulting gametophytes to compete successfully with others surrounding them for space and nutrients. This process is especially evident in those geographical areas in which density of pteridophytes and bryophytes is high, such as subtropical and tropical regions, and available niches for new individuals are relatively rare. The large number of mature individuals already present in the region will also produce an excess of spores for any given area. Due to high competitiveness in these areas the ability of a species to reproduce sexually will depend originally on the adaptiveness of its spores and gametophytes and their ability to survive to maturity. Initially, spore viability length, rapidity of germination, and rate of gametophyte growth are primary factors of competition after dissemination. These factors are controlled by both intrinsic properties (e.g., genotype, age, dormancy capability) and extrinsic properties (the condition of the environment of the immediate site where the spore alights, including pH of the soil, moisture, temperature, light, and proximity of competitors). It has become apparent by this study as well as from a compilation of information from the literature that morphological attributes of the spore, especially presence or absence of chlorophyll, and their physiological consequences have an effect on dormancy rate, rapidity of germination, and the subsequent rate of growth of the young gametophyte.