Systematic Inferences from Spore and Stomate Size in the Ferns

Abstract

Size of equivalent cells has traditionally been assumed to be constant within species and variable between species. Systematists have commonly measured cells with constant form, such as spores and stomates, as a means of distinguishing species and hybrids in polyploid complexes, since the best known factor determining cell size is ploidy level (Stebbins, 1950). Preliminary hypotheses about polyploid evolution can be generated directly from herbarium specimens based on spore and stomate measurement data. The first evidence of a relation between cell size and polyploidy in the ferns was Lawton's 1932 demonstration that prothallial cells, lower epidermal cells, and stomates in induced apogamous races of Dryopteris marginalis and Woodwardia virginica vary directly with ploidy level. Butters and Tryon's 1948 work on Woodsia x abbeae included epidermal cell and annulus cell measurements that corroborated data from spore germination experiments to document an instance of somatic autopolyploidization. More recently a considerable body of evidence has been assembled supporting the contention that spore size is related to ploidy level in the ferns. Manton (1950) noted that Vancouver Island Cystopteris fragilis (n = 84) has smaller spores than Swiss Cystopteris alpina (n = 126). Hagenah (1961) postulated a polyploid series in species of Cystopteris based on spore-size measurements. In a monograph of Cystopteris, Blasdell (1963) concluded that it was possible to infer ploidy level from spore size in that genus. Blasdell assigned ploidy levels to cytologically unknown components of polyploid complexes based only on spore size (see Lovis, 1977 for a critique). Wagner (1966) demonstrated that there were two varieties of Gymnocarpium dryopteris: a larger-spored typical variety, which is tetraploid, and a smaller-spored variety disjunctum, which is diploid. Schneller (1974) recently provided an analysis of ploidy level and spore size in the Dryopteris filix-mas group of Europe. He concluded that ploidy and spore size are positively correlated for diploid to pentaploid cytotypes in a closely knit phylogenetic group. In an exhaustive analysis of spore features of northeastern North American Isoetes, Kott and Britton (1983) provided new insight into spore-size variability. Whereas megaspore variation within sporangia of a single plant, between sporangia of a single plant, and between plants of a population was found to be more or less equivalent and negligible, variation between populations of a species was demonstrated to be twice that between plants of a population. Microspores were found to be slightly less variable. Kott and Britton (1983) suggested that