Walking through the University of British Columbia Botanical Garden’s Physic Garden, I happened to spot a patch of Asplenium scolopendrium gleaming in the sunshine. The light was making the dark centipede-like pattern of the mature sori stand out against its leaves. I was glad that I managed to take a photo that captured the interplay of light, sori, and green leaf that I saw on that warm summer day.
Asplenium scolopendrium, or hart’s tongue fern, is an evergreen species that grows in lime-rich, rocky substrates. Its simple, undivided fronds give it an unusual appearance for a fern. In a 2010 Botany Photo of the Day post, Eric La Fountaine explains that there is a North American as well as a European variety of this species. In Canada, Asplenium scolopendrium var. americanum is considered a species of special concern by the Species at Risk Act. The Canadian population is found only in the province of Ontario. Asplenium scolopendrium var. americanum also grows in the eastern USA and northern Mexico. The European Asplenium scolopendrium var. scolopendrium is widespread, and can often be found growing in the crevices of old walls.
I find the sori the most fascinating part of this plant species. In hart’s tongue fern, clusters of spore-producing structures are arranged in paired diagonal lines on either side of the fern’s midrib. Like most fern species, Asplenium scolopendrium produces only one type of spore, i.e., it is homosporous, with each spore having the potential to grow into male, female, or bisexual gametophytes. Homosporous species have an advantage when colonizing sites far from their progenitors, since they do not require a male and female spore to land and grow next to one another. Homosporous species may reproduce through the self-fertilization of a single gametophyte (intragametophytic selfing, or just selfing). This strategy poses its own difficulties, particularly for diploid species such as Asplenium scolopendrium var. scolopendrium. Fern gametophytes contain only a half-set of chromosomes, so when selfing occurs in diploid species, the resulting sporophytic generation is completely homozygous (the alleles at any given locus are identical). Groot et. al. (2012) describe diploid selfing as “an extreme case of inbreeding”.
Homosporous ferns may also reproduce through intergametophytic selfing, or the union of gametes from two gametophytes from the same sporophyte (like self-fertilization in flowering plants). Finally, homosporous ferns also reproduce through out-crossing, when gametes originating from two sporophytes unite. In the article, Mixed mating system in the fern Asplenium scolopendrium: implications for colonization potential, Wubs et. al. sought to understand the relative importance of each of the above methods of reproduction to the success of Asplenium scolopendrium. The botanists explain that European populations of Asplenium scolopendrium are able to reproduce through selfing, establishing colonies at great distances from their parent plants. Outcrossing, however, produces plants with greater fitness. Once genetically-different gametophytes establish within a colony, they are able to outcross with the original population and establish a higher level of genetic diversity overall. Wubs et. al. describe Asplenium scolopendrium‘s ability to reproduce through selfing as well as out-crossing a “mixed mating system”, and hypothesize that this system allows the species to achieve high success in colonizing new sites. It is unclear whether the tetraploid Asplenium scolopendrium var. americanum also uses the mixed mating strategy.