Selaginella (Spikemoss): Morphology, Reproduction, Uses
Selaginella, commonly known as spikemoss or small club moss, is a fascinating genus that represents the sole surviving member of the ancient vascular plant family Selaginellaceae and the order Selaginellales. Belonging to the Lycophyta division, *Selaginella* is a primitive group of vascular plants that reproduces by spores, distinguishing it from seed-bearing plants and true mosses, which are non-vascular. The genus is remarkably diverse, encompassing approximately 700 species with a worldwide distribution, though they are most abundant in the moist, shady environments of tropical rainforests. With an estimated evolutionary age of around 350 to 400 million years, *Selaginella* is considered one of the oldest known groups of extant vascular plants.
Spikemosses are typically herbaceous plants with a variety of growth habits, ranging from creeping and mat-forming to sub-erect, climbing, or erect forms. This morphological diversity allows them to thrive in varied habitats, from humid jungle floors to dry, xerophytic conditions on rocks and sandy soil. The existence of species like the famous *Selaginella lepidophylla*, the “resurrection plant,” highlights their incredible adaptation to withstand extreme drought, demonstrating a unique ability to revive after periods of complete dehydration.
Morphological Characteristics of Selaginella
The sporophyte (the dominant plant body) of *Selaginella* exhibits several distinct anatomical features, optimized for a terrestrial, vascular lifestyle. The Stem structure is generally herbaceous and solid, displaying a branching pattern. Stems can be prostrate (creeping), sub-erect, or erect, and are typically green and smooth (glabrous). Erect species, categorized under the section *Homoeophyllum*, have radially constructed stems, while creeping or dorsiventral species belong to *Heterophyllum*. In certain species like *S. umbrosa*, the stem may be reddish, and some may bear unicellular hairs.
Leaves in *Selaginella* are termed microphylls. They are small, simple, and scale-like, a characteristic of lycophytes, each possessing only a single, unbranched vein. Leaf arrangement can be spiral or opposite, depending on the species. A crucial distinction is made between isophyllous and heterophyllous species. In the subgenus *Homoeophyllum*, leaves are uniform in size and spirally arranged, forming a dense covering. Conversely, in the subgenus *Heterophyllum* (dorsiventral species), leaves are dimorphic, arranged in pairs with two small leaves on the dorsal side and two larger leaves on the ventral side of the stem, often giving the plant a flat, fern-like appearance. The presence of a well-developed cuticle helps to reduce water loss.
A unique and defining feature of *Selaginella* is the Ligule. This is a small, membranous, tongue-shaped, wedge-shaped, lobed, or fringed outgrowth found on the adaxial (upper) surface of the microphyll, near its base where it attaches to the stem. The ligule is embedded in a pit-like structure called the ligule pit and is believed to play a protective role, particularly for the young sporangia, and may also be involved in water secretion. The ligule develops quite early during the leaf’s ontogeny.
Roots are adventitious, originating endogenously, and exhibit dichotomous branching. They are crucial for water and nutrient absorption and plant stability. Uniquely, the roots develop from special leafless, colourless, cylindrical structures called Rhizophores. Rhizophores emerge from specialized regions (angle meristems) between the stem branches and grow positively geotropically (downward). Upon touching the soil, the rhizophore tip develops a tuft of adventitious roots, which have root caps and root hairs. Rhizophores serve as anchors, for feeding, and also facilitate vegetative propagation when stem fragments break off and root. Under certain conditions, they can even develop into leaf-bearing shoots, demonstrating characteristics of both a stem and a root.
Reproduction in Selaginella
*Selaginella* reproduces both sexually and vegetatively. Its sexual reproduction is a key evolutionary milestone because it is **heterosporous**, meaning the plant produces two functionally and morphologically distinct types of spores: microspores (male) and megaspores (female). This separation of spore types cuts down on self-fertilization and is a vital evolutionary step toward seed production.
These spores are produced within specialized spore-bearing leaves called sporophylls, which are typically aggregated at the tips of the branches to form a compact, cone-like structure known as a Strobilus (or sporangiferous spike). The strobili are generally terminal in position, although sometimes the axis can proliferate and overgrow into a normal vegetative shoot. Microsporophylls bear microsporangia, which produce numerous, small, haploid microspores. Megasporophylls bear megasporangia, which usually produce only four large, functional haploid megaspores. The sporangia may be bisporangiate (bearing both types of spores) or, less commonly, unisporangiate within a single strobilus.
The large Megaspores contain stored food because the female gametophyte develops and germinates entirely *within* the megaspore wall, unable to photosynthesize. This female gametophyte then exposes the egg cell. The dispersed Microspores germinate into the male gametophyte, which produces motile sperm. When there is sufficient moisture, the sperm are released to swim to and fertilize the egg, forming a diploid zygote. The resulting embryo develops a suspensor which both transports nutrients and pushes the embryo deeper into the nutritive tissue of the female gametophyte. The retention of the female gametophyte and early embryo within the spore wall and its feeding relationship closely parallel the initial stages of seed development in higher plants.
Vegetative Reproduction in *Selaginella* is also common. The plant can easily propagate through fragmentation, where parts of the stem or branches break off and root quickly using the rhizophores to form a new plant. Certain species, such as those found in xerophytic conditions, can also form specialized underground storage structures known as tubers for survival and propagation.
Uses and Ecological Significance
*Selaginella* holds importance in several domains, ranging from ornamental gardening to evolutionary biology. Horticulturally, many species are popular as ground cover in moist, shady environments due to their dense, creeping, and mat-forming habit. Their fine, lacy, often fern-like foliage, sometimes exhibiting an attractive blue sheen, makes them desirable houseplants, especially in high-humidity settings like terrariums. Species such as *Selaginella kraussiana* are easy to propagate by division or rooting stem cuttings.
From an evolutionary standpoint, the genus is of immense significance as a model for heterospory—the production of two different kinds of spores. This reproductive adaptation is considered a critical intermediate step in the evolution of seed-bearing plants (Spermatophytes). Studying the life cycle of *Selaginella* provides essential insights into the historical divergence of major plant groups.
Perhaps the most famous species, *Selaginella lepidophylla*, the Resurrection Plant, is ecologically and scientifically notable for its extreme desiccation tolerance. This species can survive years of drought by curling into a tight ball, appearing dead, and then rapidly reviving and turning green again when moisture is introduced. This incredible physiological resilience is a subject of active research into the molecular mechanisms of anhydrobiosis, with potential applications in agriculture and medicine. Overall, *Selaginella* is a vital genus for understanding the history of vascular plants and an important member of diverse ecosystems.