Systematic Position (Source: USDA Plants Database)
Kingdom- Plantae
Sub-Kingdom- Trachaeobionta
Division- Lycopodiophyta (Lycophyta)
Class- Lycopodiopsida
Order- Selaginellales.
Family- Selaginellaceae
Genus- Selaginella
It is the only living genus in the Selaginellaceae family and comprises about 800 species.
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Habitat of Selaginella
Selaginella prefers to grow in shady and damp places and is mostly found in Tropical regions of the world. They are rare in the temperate zones and almost absent in the alpine zone. Species like Selaginella lepidophylla are xerophytic and S.oregana is epiphytic.
Habit and Morphology of Selaginella
Many species of Selaginella are herbaceous perennials; however, some species like S. pygmaea are annuals. Most species (S. kraussiana) are dorsiventral and grow prostrate, a few are radial and grow erect (S. selaginoides). Some species are rarely scandent and sub-erect (S. wildenovii). Some dorsiventral species like S. umbrosa are caulescent with erect stems from creeping rhizomes.
Hieronymus (1902) divided the genus into two sections-
Homeoplyllum
This includes all the species having isophyllous leaves that are spirally arranged. They are all monostelic. It is further divided into two sub-sections:
- Cylindrostachya– It includes species with isophyllous leaves in which the sporophylls are spirally arranged. Examples are S. selaginoides, S. spinulosa, etc.
- Tetragonostachya– It includes species with isophyllous leaves in which the sporophylls are arranged in four vertical rows giving the strobilus a four-angled appearance. Examples are S. pygmaea, S. uliginosa, etc.
Heterophyllum
It includes the majority of species that possess dorsiventral symmetry, and anisophyllous leaves. The leaves are arranged in four longitudinal rows along the stem. The two rows of smaller leaves are attached to the upper side and the two rows of larger leaves are attached to the lower side of the stem. Examples are S. kraussiana, S. martensii etc.
Stem of Selaginella
The stem is herbaceous, branched, solid, and maybe prostrate, sub-erect, caulescent, climbing, or erect. Stem is usually green, smooth, and glabrous but may be red in some species (S. umbrosa) and bear unicellular hairs (S. braunii). The branching may be dichotomous, monopodial, or pseudomonopodial which varies from species to species.
In S. selaginoides, the branching of the young sporophyte is dichotomous whereas the lateral branches arise due to monopodial branching.
Leaves of Selaginella
The leaves are microphyllous. Each leaf is traversed by a single unbranched midrib. The leaf traces do not have any leaf gaps in the stem stele. The ligule arises from the base of each leaf. They are delicate, green, ligulate, and usually glabrous, with entire or serrate margin and acute apex. In species like S.picta, they are variegated. In S. hispida, they are hairy and in S. serpens, a periodic change in leaf colour is seen. In some xerophytic species, the leaves are rough and thick.
In species having isophyllous leaves, the arrangements of leaves are always spiral whereas in species having anisophyllous leaves, the small and large leaves are arranged in four longitudinal rows. The smaller leaves are arranged in two rows along the dorsal side of the stem and the larger leaves are arranged in two rows along the ventral side of the stem.
Ligule of Selaginella
The ligule develops quite early during the ontogeny of the leaf and arises from the leaf base. It varies in shape and size from species to species. The ligule may be tongue-shaped as in S. chrysocaulos, wedge-shaped as in S. martensii, lobed as in S. caulescens, lanceolate, or may even have fringed margins as in S. cuspidata.
A mature ligule has a prominent basal portion which is known as glossopodium. It is sunk in a definite ligular pit or pocket. Its function is not clearly understood.
Rhizopore of Selaginella
It is present mostly in dorsiventral species. They are leafless and positively geotropic organs that develop from a group of meristematic cells present between the two branches of the stem. In species like S. martensii, two rhizopores arise from a single meristematic region out of which one is ventral in position and another is dorsal. The ventral grows down in the soil and bears roots at its swollen end whereas the dorsal remains short.
The rhizopore is considered a root-like structure because they are positively geotropic, bears no leaves, and the internal structure resembles that of a root i.e. the stellar organization is monostelic. However, it has some stem-like characteristics as well. They lack root hairs and root caps, arise from special meristems called the angle meristems, and are exogenous in origin. Under certain conditions, they develop into leaf-bearing shoots.
Due to these controversial evidences, Bower and Goebel considered rhizopores as organs sui generis which means rhizopores are neither roots nor shoots but are intermediate structures. However, Harvey-Gibson, Van-Tiegham, and Uphoff considered them as roots.
J.C. Schoute regarded them as stem with root-bearing functions because they are exogenous in origin and their anatomical organization is similar to that of stem. He also disagrees with Bower because according to him, sui generis is a novel structure that is not raised as a result of metamorphosis whereas rhizopore has features in common with both stem and root.
Roots of Selaginella
The roots are adventitious and originate from-
- the tip of rhizopores (Eg. S. chrysocaulos)
- the swollen base of the hypocotyl (Eg. S.selaginoides) and
- directly from the stem (Eg. S umbrosa).
They arise endogenously and branch dichotomously. In some species like S. densa, the roots arise only from the places where the stem branches. The roots have root caps and bear root hair. In S. selaginoides, an endophytic fungus has been reported from the cortical cells of the root.
Strobilus of Selaginella
The strobilus is the sporangia-bearing structure of the sporophyte. The sporangia arising from the axils of leaves are called sporophylls. The sporangia in Selaginella are of two types- megasporangia and microsporangia. The sporophylls bearing megasporangia are called megasporophylls and those bearing microsporangia are called microsporophylls. The strobilus is mostly at the terminal position but in some cases, the axis of the strobilus proliferates and overgrows to form a normal dorsiventral shoot. For example S. grandis and S. erythropus the proliferating strobilus produces a sterile shoot which again bears another strobilus at its tip.
In Selaginella selaginoides, the strobilus is cylindrical. The sporophylls are spirally arranged and are isophyllous. In S. molliceps, sporangia also develop in the axils of ordinary vegetative leaves. The sporophylls in the tetragonous strobilus may be isophyllous or anisophyllous and may be spirally arranged or decussate.
In S. chrysocaulos, the vegetative region as well as the strobili is anisophyllous. In all cases, the sporophylls are ligulate and ligule is present between the sporangium and the base of the sporophylls.
The strobilus is called the sporangiferous spike or the cone. Each sporophyll bears one sporangium in its axil or a little upwards on the stem (cauline). The strobili are usually bisporangiate i.e. bearing both megaspore and microspore. In S. atrovirdis and S. gracilis, the strobili are unisporangiate i.e. a strobilus either bears microsporangia or bears only megasporangia. Both the microsporangiate and megasporangiate strobili occur in the same plant.
Anatomy of Selaginella
The cross-section of the stem reveals the following layers–
Epidermis
It is made up of thin-walled rectangular or barrel-shaped cells and is covered by a thin layer of cuticle. The cells are colorless and there are no stomata.
Cortex
It consists of many layers of cells. The outermost layers of cells develop a thick wall in the older region and form a sclerenchymatous hypodermis. The rest of the cortex is made up of thin-walled chlorenchymatous and polygonal cells. This layer varies among the species. In S. lepidophylla most of the cortex is sclerenchymatous.
Air Space
Next to the cortex there is a large air space in the center of which the stele is suspended by means of trabeculae. Trabeculae are modified endodermal cells and possess Casparian strips.
Stele
There is a single central stele suspended in the air space by trabeculae. It varies from species to species. In S. chrysocaulos, the stele is flattened like a ribbon and is called protostele with no pith in the center. The stele consists of a pericycle next to which lies the phloem and the center is occupied by the xylem.
In S. selaginoides, the stele is star-shaped with no pith hence it is called actinostelic protostele. In S. kraussiana there are two protosteles suspended in the air space. In S. willdenovii there are three or more ribbon-like protosteles.
The cross-section of Rhizophore reveals the following layers–
The outermost layer is the epidermis whose cells are thick-walled. The cortex is extensive and clearly distinguished into the outer and inner cortex. The outer cortex is thick-walled and sclerenchymatous and the inner cortex is thin-walled and parenchymatous. Followed by the cortex lies the endodermis which is followed by a single layer of parenchymatous layer called the pericycle. The stele is typically a protostele and shows variation in different species of Selaginella.
The cross-section of the root reveals the following layers–
The root epidermis is single-layered layered covered by a thin cuticle. Root hairs are present. The cortex is extensive and usually consists of the outer sclerenchymatous cortex and inner thin-walled cortex. The endodermis is indistinct in some species but distinguishable in S. densa. In S. densa, trabeculae and air spaces are present. The trabeculae are made up of extensions of cortical cells.
The stele is protostele with exarch and monarch xylem.
The cross-section of the leaf reveals–
The transverse section of the leaf consists of the upper and lower epidermis. The stomata may be present on both the epidermal layers. The epidermal cells contain chloroplasts. Beneath the upper epidermis lies the mesophyll tissue. The cells of mesophyll tissue are thin-walled cells that are loosely arranged and enclose small or large air spaces. The mesophyll tissue contains a variable number of chloroplasts. The chloroplasts contain numerous spindle-shaped bodies that ultimately become transformed into starch grains.
The chloroplasts show grana-like structures called granoids. The vascular bundles contain a phloem that surrounds the xylem. There is no distinction between protoxylem and metaxylem. A single layer of cells completely encircles the phloem which may be regarded as a bundle sheath.
The cross-section of the ligule reveals the following layers–
A fully developed ligule consists of a distinct and hemispherical basal region where the cells are large and contain vacuolated cytoplasm. This region is glossopodium. It is surrounded by a sheath called glossopodial sheath. Next to this region, the ligule usually narrows down or broadens or is produced into finger-like processes. Depending upon the shape of the ligule the cells in the distal portion are likewise arranged. In tongue-shaped and spindle-shaped ligules the cells in the distal region are narrow and elongated.
Reproduction in Selaginella
Selaginella reproduces by the following means–
Fragmentation
It is observed in some species that grow under humid conditions. In S. rupestris, the trailing branches of the stem develop adventitious branches and later separate from the parent plant.
Tubers
Formation of tubers has been reported in species like S. chrysorrhizos. In this, the tubers are formed underground. The tubers bear rudimentary scales and appear towards the end of the growing season at the tips of underground branches that arise from the base of the stem. During unfavorable conditions, the aerial portions die and with the advent of favorable conditions, the tubers germinate into new plants.
Resting buds
This type of vegetative reproduction is seen in S. chrysocaulos. They develop at the ends of some aerial branches and are very compact structures. The leaves in this region are closely arranged and overlap each other covering the growing point. The buds give off rhizophores that bear roots at their tips. The resting buds grow into new individuals during favorable conditions.
Reproduction by Spores
Selaginella is heterosporous and the spores are of two types- megaspore and microspore.
Megaspores and microspores are produced in megasporangia and microsporangia respectively, which arise on the axils of megasporophyll and microsporophyll. The sporophylls are arranged in definite loose or compact terminal structures called the strobili or spikes.
Sporangia
Microsporangia
A microsporangia may be oval, reniform, or spherical. The outline is almost smooth and regular.
It is smaller in size than the microsporangium and has multicellular stalk. The wall of the sporangium is two-layered thick.
Next to the inner wall lies the tapetum that provides nutrition. This tapetal layer is aroused from the archesporium.
The wall layers and tapetum enclose a large number of diploid microspore mother cells which later give rise to haploid microspore tetrads.
Development
The development of sporangium is essentially the eusporangiate type. In species like S. martensii, the sporangium the sporangium is cauline in origin. In S. selaginoides, the sporangium is foliar in origin.
The sporangium arises from a transverse row of cells called the sporangial initials. This initially divides by periclinal division to form outer primary jacket cells or wall cell and inner primary archesporial cell. The wall cell undergoes anticlinal and periclinal divisions to form a two-layered wall.
The archesporium divides to form a group of sporogenous cells. The outermost layer of sporogenous cells divides periclinally to form a tapetum. The tissue at the base of the sporangium divides to form the sporangial stalk. The sporogenous tissue undergoes repeated divisions to form spore mother cells or microspore mother cells.
The microspore mother cells undergo meiosis to form tetrahedral tetrads of microspores. These later separate into individual microspores. The tapetum modifies into the plasmodial fluid.
Dehiscence
The microsporangium dehisces by the appearance of the vertical slit on the apical part of the sporangial wall. The slit separates the walls and the cells in the basal region shrink due to loss of water which exerts pressure on the spores and forces them to come out from the slit.
Megasporangia
It is comparatively larger, shortly stalked, four-lobed, and maybe green cream or brown. It has a two-layered wall and the innermost layer of tapetum is also present.
Development
The development is similar to that of microsporangium up to the formation of spore mother cells. In megasporangium, only one megaspore mother cell remains functional and all the others degenerate. The functional megaspore mother cell undergoes meiosis to form a megaspore tetrad.
Dehiscence
The dehiscence is similar to microsporangium. The only difference is it scatters more violently and scatters megaspores to a greater distance than the microspores.
Gametophytic Generation
The microspores and megaspores are the pioneer structures of this generation.
Microspores
The microspores are usually tetrahedral in shape with a rounded upper end and a distinct tri-radiate mark. Every spore has a single nucleus surrounded by cytoplasm and enclosed by a spore layer wall. The outer layer is exine which is differentiated into two layers- the outer one is sexine or ectine and the inner one is endine or nexine. The inner layer is intine.
Germination of microspores
The germination of microspores has been given by Slagg (1932) for S. kraussiana. The germination is precocious i.e. it starts within the microsporangium. The haploid nuclei of the microspore divide into two daughter nuclei. One of the two daughter nuclei migrates to one side of the spore. A cell wall is laid down separating a small lens-shaped prothalial cell and the larger one is called an antheridial cell. The prothalial cell remains inactive and does not divide further. The antheridial cell divides along several planes and gives rise to antheridium.
The first division of the antheridial cell is vertical. The two cells formed again undergo transverse division. Out of the four cells formed, the two basal cells do not divide further. The upper two cells divide in such a manner that the upper wall meets the lower wall and forms a curve-shaped structure. In this stage, the microgametophyte has seven cells i.e. 6 cells formed from an antheridial cell and one prothalial cell.
Out of the last four cells formed, the two bigger one again divides by curved walls and result in the formation of eight antheridial cells arranged in such a manner that four cells in the middle separate from two either cell upper and below. The middle 4 cells divide periclinally to cut off 4 primary androgonial cells from 8 jacket cells. This is the 13-celled stage. These are 8 jacket cells, 4 primary androgonial cells, and 1 prothalial cell.
At this stage, the microgametophyte is shed from the microsporangium. In some species, the male gametophyte may release at an advanced stage.
The four androgonial cells further divide to form androcytes mother cells that metamorphose into androcytes or sperms. During the formation, the eight jacket cells undergo degeneration so that the androcytes float on the mucilage-filled cavity of the microspore.
Spermatozoid
The cytoplasm of each androcyte metamorphoses to biflagellate spermatozoid. The spermatozoid consists of three main parts- the nucleus, the vesicle, and the motor apparatus that consists of cilia cilia-bearing band or the stalk.
The spermatozoids are liberated by opening the spore wall at the tri-radiate mark. The freshly released spermatozoids have their body coiled around the lens-shaped vesicle; later the vesicle absorbs water and becomes spherical and soon set free. The stalk of the motor apparatus is situated at the anterior end of the spermatozoid. It bears two flagella out of which is at the extreme tip and the other one is a little below. Each of these is attached to a basal granule.
Morphology of prothalial cell
The single prothalial cell of the microgametophyte is regarded as equivalent to the entire gametophytic tissue of the homosporous vascular cryptogams.
Megaspores
The megaspore is larger than the microspore and is tetrahedral in shape with a distinct triradiate mark. The megaspore is also unicellular and uninucleated. The megaspore also has two walls exine and intine. The exine is differentiated into two layers ectine and endine.
Megagametophyte
The megagametophytes develop as a result of the germination of megaspores. The germination is precocious.
Campbell (1902) studied the development of megagametophytes in S. kraussiana and the following observations were made.
The first visible change is the separation of endine from ectine. Thus a space is formed which is filled with a non-stainable fluid. Then the cytoplasm forms a thin lining layer next to intine and encloses a big vacuole with a nucleus embedded in the cytoplasmic layer. The free nuclear division takes place.
The nuclei were flattened at first and later they become rounded. The evenly distributed nucleus now becomes uneven and the number increases towards its apex. At this stage, the vacuole increases in size and touches the intine. Later due to nuclear division and consequent increase in cytoplasm, the central vacuole diminishes in size.
The cell walls are laid down first around the nuclei in the apical region which leads to the formation of a single layer of cells. Later cell walls are laid down around the lower layers of nuclei forming three layers at the middle and only a single layer towards the margins. The innermost layer of the cell is in contact with the vacuole. Later the inner walls of innermost cells become thickened forming a diaphragm that separates the upper cellular region with a basal region that is yet non-cellular and has free nuclei.
The free nuclei divide and re-divide and the amount of cytoplasm increases till the central vacuole is completely obliterated. The cell walls in this region appear at different stages in different species.
The entire megagametophyte becomes cellular at one or the other stage and can be distinguished into two regions- The upper cushion of small cells or the generative region and the lower nutritive zone. The megagametophytes in this stage are released by the dehiscence of meagsporangia.
Development of Archegonia
Each archegonium develops from a superficial cell called the archegonial initial. The archegonial initial divides by a periclinal wall to form an upper primary cover cell and a lower central cell. The central cell further divides periclinally to form the upper primary canal cell and lower primary ventral cell. The primary canal cell acts as a single neck canal cell and the primary ventral cell divides to form the ventral canal cell and egg cell.
The primary cover cell divides to form four neck initials which in turn divide transversely to form eight-celled neck. These cells are arranged in two tiers of four cells each. The entire archegonium except the upper tier of neck cells is embedded in the apical cushion of megagametophyte.
Fertilization in Selaginella
The spermatozoids swim toward the archegonial neck in the presence of water. The malic acid present in the archegonial neck attracts the spermatozoids and more than one sperm may swim down the canal but only one penetrates the egg to accomplish fertilization.
Embryo
The embryo in Selaginella varies slightly within the species. The embryo development in S. martensii is discussed below.
The zygote elongates in the axis of the archegonium. The zygote divides transversely forming an upper hypobasal (suspensor) and the lower epibasal (embryonal) cell. The suspensor cell elongates and divides to form a multicellular suspensor that elongates downwards pushing the embryo deep into the thallus.
The embryonal cell divides into two equal cells by a vertical wall and again divides right angles to the first which leads to the formation of the quadrant stage. One of the cells divides by an oblique wall resulting in the formation of a shoot apical cell. The remaining cell divides transversely to form eight cells arranged in two tiers of four cells each. The cells in both the tiers divide and re-divide to form an undifferentiated mass of cells. The division is more active in the quadrant near the suspensor and forms a distinct mass of cells called a foot.
The two superficial cells of the quadrant away from the suspensor act as cotyledon initials which divide to form cotyledons. Each of these develops a ligule at its base. Meanwhile, the apical cell forms a distinct shoot apex enclosed by the cotyledons.
The superficial cell lying on one side of the foot acts as root initial which forms the primary rhizophore. Up to this, the foot absorbed the nutrients for the young embryo from the sporophyte, but by the formation of the first root, the sporophyte becomes independent. The young sporophyte with cotyledons, stem and root is an independent individual that later develops into a mature plant.
Parthenogenesis
It has been reported in S. atrovirdis. In this species, the egg develops into an embryo without fertilization. In some species, the archegonial neck remains closed and does not permit the spermatozoids to enter. Due to this, no fertilization takes place and the egg directly develops into an embryo.
Economic Importance of Selaginella
Selaginella bryopteris, commonly known as ‘Sanjeevani,’ is believed to have rejuvenating properties and is used in traditional Indian medicine. It is used in the Ayurvedic medicinal system for treating stress, fever, and jaundice, and as a general health tonic. Selaginella flabellate is used to control fever, headaches, and menstruation (Blackwood, 1953).
Selaginella involvens has been studied for its anti-inflammatory and antimicrobial effects.
Selaginella is also used as an ornamental plant so it has great importance in horticulture. Its feathery appearance and low maintenance make it popular in the horticultural field. It is also used in decorations.
Selaginella uncinata, commonly known as the ‘Peacock Fern’ due to its vibrant blue-green foliage, is a popular ornamental plant. It is widely used in terrariums and as ground cover in shaded gardens, adding aesthetic value.
S. lepidophylla is also known as the resurrection plant and is sold as a dried plant on roadsides and rejuvenates once it comes in contact with water. Various species of Selaginella are also used by greenhouses as border plants. Selaginella moellendorffii is a widely used model organism in genetic and evolutionary studies. Its relatively simple genome has been sequenced and provides insight into the evolution of land plants.
References
- Kala, C. P., & Dhyani, P. P. (2000). The Sanjeevani of Indian Himalayan Region: An overview on the ecological, ethnobotanical and pharmaceutical aspects of Selaginella bryopteris. Journal of Ethnopharmacology, 71(3), 245-256.
- Richards, P. W., & Evans, A. (1992). Selaginella: A plant of many uses. Horticultural Science, 27(6), 585-590.
- Smith, A. R., & Stewart, N. C. (2008). Economic importance and biotechnological applications of Selaginella species. Journal of Biotechnology, 26(3), 123-130.
- Jena, R. C., & Nayak, S. (2012). Anti-inflammatory and antimicrobial properties of Selaginella involvens. Journal of Medicinal Plants Research, 6(29), 4437-4443.
- Pteridophyta by B.R. Vashishta, A.K. Sinha, Adarsh Kumar (S.Chand & Company Ltd)
- Hait, G., Bhattacharya, K., & Ghosh, A. K. (2012). A textbook of Botany, Volume I.
- Puri, P. (1980). Bryophytes and Pteridophytes. Atma Ram and Sons.
- Sen, U., & Ghosh, B. (1960). Studies on the Indian Species of the Genus Selaginella. Journal of the Indian Botanical Society, 39(2), 145-162.
- Admin. (2023, July 28). Selaginella – NEET Biology Notes. BYJUS. https://byjus.com/neet/selaginella/
- Saraswat, K. (2024, March 26). Selaginella – Classification, characteristics, and life cycle. PHYSICS WALLAH. https://www.pw.live/exams/neet/selaginella/
- Selaginella. (n.d.). https://sathee.prutor.ai/neet-biology/selaginella/