Selaginella is a pteridophyte. It is also called spikemoss or club moss. It is the largest and the only living genus of the family Selaginellaceae. It contains more than 800 species distributed all around the world with the highest diversity found in the tropical regions.
They are seedless vascular plants. They are mostly found in shady areas, some species are also present in seasonal dry or xerophytic conditions. They are found on tree trunks, rocks, forest floors, etc.
Some of the common species of Selaginella are:
- S. kraussiana (golden clubmoss)
- S. erythropus (ruby-red spikemoss)
- S. lepidophylla (resurrection plant)
- S. uncinata (peacock moss)
The main plant body is a sporophyte. It is a vascular plant and is differentiated into root, stem and leaves. Some of the important features are:
- It is an evergreen, profusely branched and delicate herb.
- Primary roots are short-lived. The adventitious roots are present at the tips of rhizophore.
- Rhizophores are leafless colourless branches that develop from the prostrate stem and grow downwards. When it reaches the soil, adventitious roots develop at the tip of rhizophores.
- Stem is green, dichotomously branched, erect or prostrate with erect branches.
- A single apical cell is generally present at the growing tip of the stem.
- Leaves are small with pointed tips. They bear scaly leaves.
- Leaves contain unbranched midrib.
- The subgenus homophyllum has spirally arranged leaves of the same kind.
- The subgenus heterophyllum consists of two types of leaves. Small leaves are present at the dorsal side and bigger leaves are present at the ventral side of the stem as pairs.
- Sporophylls or spore bearing leaves are similar in structure to the ordinary leaves but are present in a cluster known as strobilli. They are usually present at the tip of branches and bear sporangia.
- Ligules are small outgrowth present at the adaxial side of the leaf at the base.
- Epidermis is without hair and stomata. It is surrounded by a cuticle.
- Epidermis is followed by a well defined cortex region. It is parenchymatous or is differentiated into outer sclerenchymatous and inner parenchymatous regions.
- Stele is generally protostelic, where xylem is surrounded by phloem cells. The number of steles varies in different species.
- Stele is surrounded by a pericycle that is single layered. Pith is generally absent.
- Xylem lacks vessels and phloem lacks companion cells.
- Epidermis is the outermost covering. Root hairs arise from epidermis. It is surrounded by a thin layer of cuticle.
- Cortex is either entirely made up of parenchymatous cells or both sclerenchymatous hypodermis and parenchymatous inner cortex.
- Endodermis may or may not be very well defined.
- Pericycle is one to three layered and is made up of parenchyma.
- Stele is protostele with exarch (protoxylem is present at the periphery) and monarch xylem.
- The structure of rhizophore is similar to that of roots. The epidermis of rhizophore lacks stomata and hairs.
- Stele is a protostelic. Xylem is exarch or measarch.
- Epidermis is single layered. Stomata are mostly present in the lower epidermis.
- Epidermis cells have chloroplasts.
- Mesophyll constitutes the part between the upper and lower epidermis. It is made up of parenchyma. The number of chloroplasts in the mesophyll varies from species to species.
- In some species, the mesophyll cells may differentiate into upper palisade and lower spongy tissues.
- A single median vascular bundle is present. It contains xylem tracheids surrounded by sieve elements of phloem. Phloem is surrounded by a single layered bundle sheath.
Reproduction and Life Cycle
Selaginella shows vegetative as well as sexual reproduction. Vegetative reproduction occurs through tubers, buds and fragmentation. Sexual reproduction occurs through spore formation.
- Adventitious branches separate and develop into new plants.
- Vegetative reproduction through tubers occurs at the end of the growing season. Tubers generally develop at the apex of aerial branches and develop under favourable conditions to give rise to a new plant.
- The resting buds develop at the tip of some aerial branches. They withstand the unfavourable conditions and develop rhizophore in the favourable conditions.
Selaginella is heterosporous. It produces two kinds of spores. The smaller microspores give rise to male gametophytes and the larger megaspores give rise to female gametophytes.
Microspores develop in microsporangia and megaspores develop in the megasporangia. They are borne on microsporophylls and megasporophylls, respectively. The aggregation of microsporophyll and megasporophyll forms a structure called strobilus.
Strobilus is generally bisporangiate but in some species it is monosporangiate, i.e. having either microsporophyll or megasporophyll.
Cells of sporogenous tissue present inside the microsporangia give rise to microspore mother cells. Each microspore mother cell divides by meiosis to produce microspore tetrads. Microspores are haploid.
Sporogenous cells inside the megasporangia develop into megaspore mother cells. Except one megaspore mother cell, all disintegrate. The functional megaspore mother cell divides by meiosis to give rise to four haploid megaspores. Some species produce only one megaspore.
Microspores germinate into male gametophytes. On maturity, microsporangium dehisces and spores are carried away by wind. The germination of microspores starts in the microsporangia. The spores are generally liberated at 13-cell stage, which consists of 1 prothallial cell + 4 androgonial cells + 8 jacket cells. The primary androgonial cells divide and develop into antherozoids. Antherozoids are biflagellate and swim towards archegonium.
Megaspores germinate into female gametophytes. The development of female gametophytes occurs inside the megasporangium. Female gametophytes are liberated after fertilisation or after archegonium formation. After liberation, it attaches to the suitable substratum and rhizoids are formed that help in anchoring and absorption of water.
Archegonium is embedded in the gametophytic tissue.
Anterozoids swim and reach towards archegonium. Water is essential for fertilisation. Nucleus of antherozoids and egg fuses to give rise to a diploid zygote or oospore after fertilisation. Oospore is the initial sporophytic generation. The oospore divides and forms a multicellular structure that differentiates into various tissues such as stem, root, cotyledons, rhizophore, etc.
In some species oospore development completes in the megasporangium and falls on the ground only after formation of a seedling with primary roots.
Selaginella like other pteridophytes show alternation of generations. The life cycle alternates between diploid sporophyte and haploid gametophyte generation. This type of life cycle is called a haplo-diplontic life cycle.
The sporophyte is the dominant phase in the life cycle. The main plant body is a diploid sporophyte and is differentiated into true roots, stem and leaves.
Selaginella is heterosporous, i.e. it produces two kinds of spores; microspores and megaspores. Spores are haploid and produced by meiosis in the spore mother cells. Spores germinate into male and female gametophytes. They are small but free-living, multicellular and photosynthetic.
Male and female gametophytes bear antheridia and archegonia, respectively. The fusion of male and female gametes results in the formation of a diploid zygote that develops into a well differentiated sporophyte.
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