Selaginella 

Selaginella, known as spike moss or club moss plant. Selaginella flourish in various ecological niches is evident across its 700 known species, each with specific habitat preferences.

1.0Classification of Selaginella

2.0Habitat of Selaginella

Selaginella are a highly adaptable genus thriving in diverse global ecosystems. From tropical rainforests to temperate forests, arid deserts, and high-altitude alpine regions, these plants display remarkable resilience. Some species adopt an epiphytic lifestyle, growing on other plants for support without extracting nutrients. Thriving in moist and shaded environments, 

Selaginella, known as spike moss plant or club moss plant Selaginella's ability to flourish in various ecological niches is evident across its 700 known species, each with specific habitat preferences. Within this genus, species can exhibit different types of leaves, and two terms often used to describe these variations are homophyllous and heterophyllous.

• Homophyllous (Homoeophyllum): In homophyllous species of Selaginella, all the leaves are similar in shape and size throughout the plant. This means that there is little to no differentiation in the appearance of leaves along the stems or branches. The uniformity in leaf morphology is a characteristic feature of homophyllous Selaginella species.
• Heterophyllous (Heteroeophyllum): In heterophyllous species, there is a noticeable difference in the size, shape, or structure of leaves on the same plant. Typically, the leaves at different parts of the plant or at different stages of development exhibit variations. This heterophylly can serve various functions, such as adaptation to different environmental conditions or roles in reproduction.

3.0Morphology of Plant Selaginella (Sporophyte)

Stem : 

The stem of the Selaginella plant is typically herbaceous and displays a branching pattern, although some species may have an unbranched structure. It can take on various forms, being prostrate, sub-erect, caulescent, climbing, or erect. Erect species fall into the radially constructed category and are classified under the section Homoeophyllum, while the others are dorsiventral and are categorized in the section Heterophyllum. The stem is commonly green, smooth, and devoid of hair (glabrous).

Root : 

Selaginella plants develop adventitious roots, which initiate from the tips of rhizophores, the swollen bases of hypocotyls, or directly from the stem. These roots originate endogenously and exhibit dichotomous branching. In certain instances, roots emerge solely from points where the stems branch. Featuring root caps, these roots also bear root hairs, contributing to the plant's ability to absorb water and nutrients from the soil. 

Leaves : 

Selaginella leaves, called microphylls, have distinct features. They arrange in spiraled or opposite patterns along stems, varying by species. Microphylls are small and simple, with a single vein, differentiating them from more complex leaves in other plant groups. Leaf size and shape vary, some uniform (homophyllous), others with variations on the same plant (heterophyllous). Leaves are generally smooth and may be glabrous, with exceptions in texture. Margins can be entire or have small teeth. Well-developed cuticles reduce water loss, adapting to diverse environments.

Ligule : 

In Selaginella, ligules are small, membranous structures that are found on the upper surface of the microphylls (leaves). The term "ligule" is used to describe the appendage or outgrowth at the base of the upper surface of the microphyll, where the leaf is attached to the stem. Ligules in Selaginella play a role in protecting the sporangia, which are structures that produce spores for reproduction.

Rhizophore : 

Rhizophores in Selaginella are specialized structures essential for the plant's reproduction and adaptability. These modified stems emerge from the lower surface of the main stems, acting as anchors and sites for vegetative reproduction. 

Rhizophores produce roots that absorb nutrients and provide stability. They are crucial for Selaginella's ability to propagate vegetatively, allowing it to colonize new areas and adjust to changing environments. The tips of rhizophores are vital for soil penetration and initiating new root systems.

4.0Anatomy of Selaginella Plant (Sporophyte)

Stem : 

Epidermis : it is made up of a single layer of thin-walled rectangular or bareela- shaped cells and covered by a thin layer of cuticle. There is no stomata. 

Cortex : The cortex is composed of numerous cell layers. In the mature sections of the stem, typically 2 to 4 outermost layers of cells undergo wall thickening, forming a sclerenchymatous hypodermis. The remaining cortex consists of thin-walled chlorenchymatous and polygonal cells, enclosing small intercellular spaces. Next to the cortex there is a large air space in the center of which the stele region is suspended by means of trabeculae, these are the modified cells of endodermis which possess a casparian strip.

Stele or the Vascular region: The stele is suspended in an air space by trabeculae, featuring a flattened, ribbon-like protostele without a central pith. The pericycle, a single layer of thin-walled cells, surrounds the inner vascular tissues. Next to the pericycle is the phloem, consisting of parenchyma cells and one or two layers of sieve cells; companion cells are absent. The central xylem is encircled by the phloem, with large polygonal sieve cells visible. The xylem lacks a pith and consists of metaxylem and Protoxylem, 

Leaves : it is dorsiventral and consists of distinct upper and lower epidermis that are single layered. In the majority of the species stomata are restricted to the lower epidermis and the vascular bundle is  very simple surrounded  by the phloem. Xylem has tracheids only. A single layer of cells completely surrounds the phloem which is regarded as bundle sheath cells. 

Root : The root epidermis in Selaginella is a single-layered structure covered by a thin cuticle, and it features the presence of root hairs. The cortex is wide and extensive and usually consists of an outer sclerenchymatous cortex of 3 to 5 or more layers in thickness.  Next to the endodermis is a single layer of pericycle.the stele is prostele with exarch and monarch xylem.  

5.0Reproduction in Selaginella

Selaginella reproduce by both vegetative and sexual reproduction : 

Vegetative Reproduction

Fragmentation : adventitious branches develop and separate from the parent plant and grow into separate individual plant. 

Tubers : Tubers are formed underground at the tips of branches arising from the stem base. During adverse conditions, aerial parts of the plant wither, and tubers aid in perennial survival. When conditions improve, tubers germinate, giving rise to new Selaginella plants.

Resting buds : these are reported to develop at the tip of the areal branch. These resting buds survive the adverse conditions. When favorable conditions return the bud gives off rhizosphores that bear roots and fix them to the soil. 

Sexual Reproduction

Selaginella is characterized by heterospory, producing two types of spores: microspores and megaspores. These spores are formed within microsporangia and megasporangia, which emerge in the axils of microsporophylls and megasporophylls. These sporophylls can be either cauline or foliar in nature. The arrangement of sporophylls forms distinct terminal structures known as strobili or spikes, which can be either loosely or compactly arranged. Strobili develops at a specific stage of maturity in the Selaginella life cycle.

Inside the microsporangia and megasporangia sporogenous cells develop into microspore mother cells and megaspore mother cells. 

Each microspore mother cell is functional and undergoes meiosis to form four tetrahedral haploid spores. While in megasporangia only one megaspore mother cell is functional, and the other degenerate. This functional megaspore mother cell divides meiotically and produces 4 haploid megaspores. 

6.0Gametophytic Generation of Selaginella  

The microspore and megaspore germinate and start the gametophytic generation of selaginella.

Male gametophyte : 

Germination of microspore starts in the microsporangia and it liberates at 13 celled stage. These cells are 8 jacked cells, 4 primary androgonial cells and one prothallial cell. The rest of the development proceeds when they fall on suitable substratum. During the future development of the 13 celled stage, primary androgonial cells divide and give rise to 128-256 androcytes which future metamorphosis into a biflagellate spermatozoid. 

Female gametophyte : 

Germination of megaspore starts in megasporangia. The liberation of the megaspores enclosing the female gametophytes usually takes place after the formation of the archegonia.They fall down on the soil and germinate. A few rhizoids develop and fix the female gametophyte to the soil and absorb moisture. The mature archegonia are embedded in the gametophytic tissue. 

Fertilization : 

water is essential for the fertilization process in species of selaginella. The spermatozoids swim in water and reach to the open archegonial neck and fertilize the egg and zygote or oospore is formed. Zygote or oopsore is the pioneer structure of the sporophyte generation and divides and embryos are formed. The embryo gives rise to the sporophytic plant. 

7.0Common Species of Selaginella

1. Selaginella apoda - Spike moss 
2. Selaginella bryopteris - Sanjeevani 
3. Selaginella lepidophylla - Resurrection plant 
4. Selaginella canaliculata - Club moss 
5. Selaginella  erythropus - Ruby-red spikemoss
6. Selaginella  kraussiana - Golden clubmoss
7. Selaginella willdenowii - Peacock moss 

8.0Life Cycle of Selaginella