Selaginella - Pteridophytes

Selaginella

Division – Lycophyta

Class – Ligulopsida

Order – Selaginellales

Family –Selaginellaceae

Genus – Selaginella

Selaginella is commonly called ‘spike moss’. They are distributed in humid temperate and tropical rain forests. Selaginella rupestris and Selaginella lepidophylla are Xerophytic.

Selaginella kraussiana, Selaginella chrysocaulos, Selaginella megaphylla are some common species. In few Selaginella species during dry season the entire plant body gets curled and become fresh, green when moisture is available. Due to this they are called Resurrection plants. Example S. lepidophylla

External morphology

Habit

The plant body of Selaginella is sporophyte (2n) and it is differentiated into root, stem, and leaves (Figure 2.25). There exist variations in the habit of Selaginella. Some species possess prostrate creeping system (S. kraussiana); suberect (S. rupestris); erect (Selaginella erythropus); Climbing (Selaginella alligans). S. oregana is an epiphyte. Most of the species are perennials. on the basis of structure of stem and arrangement of leaves, Selaginella is divided into two sub genera namely Homoeophyllum and Heterophyllum.

Homeophyllum include species with erect stem and spirally arranged leaves. (Example: S. upestris and S. oregana). Heterophyllum include species with prosrate stem with short erect branches and dimorphic leaves (Example: S. kraussiana and S. lepidophylla).

Root

Primary roots are short lived and the adult plant has adventitious roots. The root may arise at the point of dichotomous branching or knot like swelling present at the basal portion of the stem. Roots are endogenous in origin.

Rhizophore

In many species long, cylindrical, unbranched and leafless structures arise from the lower side of the stem at the point of dichotomy called rhizophores. They grow vertically downwards and produce tufts of adventitious roots.

Stem

The stem may be erect, dichotomously branched or prostrate with lateral branching. The prostrate stem is dorsiventral.

Leaves

The leaves are microphyllous, sessile and simple. A single midvein is present in the leaves. The vegetative leaf as well as the sporophyll bears a small membranous tongue like structure on adaxial surface called ligule. The basal part of the ligule possess a hemispherical mass of thin walled cells called glossopodium. The function of ligule is not known, but it is viewed to be associated with water absorption, secretion and prevent dessication of shoot. The members belonging to Homeophyllum type possess same type of leaves spirally arranged on the stem whereas the Heterophyllum type have two types of leaves- two dorsal rows of small leaves(Microphylls) and two ventral rows of large leaves(Megaphylls).

Internal structure

Root

The transverse section of the root reveals an outermost layer called epidermis. It is made up of tangentially elongated cells. The cortex is homogeneous made up of thin walled parenchyma . The innermost layer of cortex is called endodermis. The stele is a protostele, monarch and xylem is exarch (Figure 2.26).

Rhizophore

The outermost layer of Rhizophore is the epidermis. It is single layered and is covered with a thick cuticle. The cortex is differentiated into outer scelrenchymatous and inner parenchymatous layers.

The innermost layer of cortex forms endodermis. The stele is a protostele Figure 2.27. It is monarch and exarch but it is centrifugal in S. kraussiana and crescent shaped in S. atroviridis.

Stem

The anatomy of the stem reveals the presence of epidermis, cortex and stelar region (Figure 2.28).

The epidermis is parenchymatous and is covered with a thick cuticle. The cortex is parenchymatous with cells arranged without intercellular spaces. A sclerenchymatous hypodermis is noticed in Selaginella lepidophylla. The presence of radially elongated endodermal cell s called trabeculae is the characteristic feature of Selaginella. The casparian strips are found on the lateral walls. The rapid stretching of the innermost cortical cells in comparison with stele results in air spaces and stele appears to be suspended in air space with the help of trabeculae. The stele is a protostele and exarch. A variation in number of steles is found. It may be monostelic (S. spinulosa); distelic (S. kraussiana )or polystelic (S. laevigata). The xylem is monarch(S. kraussiana) or diarch (S. oregana) . Tracheids are present but vessels are also noticed in S. densa and S. rupestris.

Leaf

The leaf shows upper and lower epidermis. The epidermal cells have chloroplast. Stomata occur on both surfaces. The mesophyll is made up of loosely arranged thin walled cells with intercellular spaces. There is a median vascular bundle surrounded by a bundle sheath. In vascular bundle xylem is surrounded by phloem.

Reproduction

Selaginella shows both vegetative and asexual modes of reproduction. 

Vegetative reproduction

Selaginella reproduces vegetatively by fragmentation, bulbil formation, tuber formation and resting buds.

Sexual reproduction

During sexual reproduction spores are produced (Figure 2.29). Selaginella is heterosporous and produces two types of spores namely microspores in microsporangium and megaspores in megasporangium. 

The sporangia are borne singly in the axils of microsporophyll and megasporophyll respectively. The sporophylls are arranged spirally around a central axis and aggregate to form strobili or cones. Variations in the distribution of microsporangia and megasporangia among the species are seen. In S. selaginoides and S. rupestris megasporangia are present in the basal part of the cone. S. kraussiana possesses a single megasporangium at the base of the strobilus. In S. inaequifolia one side of the strobilus bear only megasporangia and other microsporangia. Separate strobili for microsporangia and megasporangia are present in S. gracilis. and S. atro-viridis.

The development of sporangium is of eusporangiate type. The sporangial initial divides periclinally to form outer jacket initials and inner archesporial initials. The archesporial initials by repeated anticlinal and periclinal divisions form sporogenous cells. Microspore mother cells of microsporangiumundergoreductiondivision to produce halpoid microspores. Similarly the megaspore mother cell undergoes reduction division to produce 4 haploid megaspores. The microspore and megaspore represent the male and female gametophyte and germinate inside the sporangium. The microspores produce biflagellate antherozoids. Archegonia develop in the megaspore. The antherozoids swim in water and reach the archegonium. Fertilization brings the fusion of male and female nucleus which result in the formation of a diploid zygote. The diploid zygote represents the first cell of sporophyte.

It undergoes several mitotic division to form embryo. The embryo develops into a mature sporophytic plant.

In the life cycle of Selaginella alternation of sporophytic and gametophytic generation is present (Figure 2.30).