The genus Selaginella, commonly known as spikemoss or lesser clubmoss, is a group of vascular plants belonging to the ancient division Lycophyta. These plants resemble mosses but possess true vascular tissues, representing one of the oldest lineages of living vascular plants on Earth. The direct answer to whether Selaginella is homosporous or heterosporous is that it is definitively heterosporous. This characteristic of producing two different types of spores was a major evolutionary step in the plant kingdom.
Defining Spore Production Types
Plant life cycles involve an alternation of generations, switching between a spore-producing sporophyte generation and a gamete-producing gametophyte generation. Homospory, or isospory, is the ancestral condition where a plant produces only one size and type of spore. These uniform spores germinate into a single, typically bisexual gametophyte that carries both male and female reproductive structures. Most ferns and the closely related genus Lycopodium are examples of homosporous plants.
In contrast, heterospory is a reproductive strategy where the plant produces two distinct types of spores that differ significantly in size. These are the small, numerous microspores and the large, few megaspores. This size difference corresponds to a separation of sexes in the next generation. Microspores develop into male gametophytes, while megaspores develop into female gametophytes.
The Mechanism of Heterospory in Selaginella
The heterosporous nature of Selaginella is manifested in its specialized reproductive structure called the strobilus, or cone, which forms at the tips of the branches. This cone is an aggregation of modified leaves known as sporophylls. Selaginella possesses two types of sporophylls: microsporophylls and megasporophylls.
Each sporophyll bears a single sporangium in its axil. The microsporophylls bear microsporangia, which produce a large number of small microspores. Conversely, the megasporophylls bear megasporangia, which typically produce only four much larger megaspores. In a single Selaginella strobilus, both types of sporangia are usually present, demonstrating the plant’s bisporangiate nature.
The development of the gametophytes in Selaginella occurs within the confines of the original spore wall, a process known as endosporic development. The microspore germinates to form a highly reduced male gametophyte, which consists mostly of an antheridium (male reproductive organ). The megaspore develops into a female gametophyte, which remains within the megaspore wall and contains the archegonia (female reproductive organs). This retention of the female gametophyte, along with its nutrient-rich megaspore, represents an early form of parental investment.
The Evolutionary Significance of Heterospory
The evolution of heterospory in Selaginella and other plant groups marks a turning point in plant history. By producing two different spore sizes that lead to separate male and female gametophytes, heterospory prevents self-fertilization, promoting outcrossing and increasing genetic diversity. This reproductive strategy provided an adaptive advantage, particularly in the competitive and shaded environments of the Devonian period, where larger megaspores could provide more resources for the developing new plant.
The connection between heterospory and the seed habit found in gymnosperms and flowering plants is important. Selaginella exhibits key steps toward seed evolution: the production of two spore types, the endosporic development of the gametophytes, and the retention of the female gametophyte within the megaspore wall. In seed plants, the megaspore is permanently retained within the megasporangium, which then becomes enclosed by protective layers called integuments. Selaginella’s mechanism of retaining the megaspore and the developing female gametophyte demonstrates a pathway toward the development of true seeds.