Selaginella: Tale of spikemosses

0
54
http://www.onlinereptileshop.co.uk/reptile-products/Live-Plants/Prorep-Live-Plant-Selaginella-Sp-85cm-Pot-8259.html#sthash.IfKel3A6.dpbs

Selaginella sp.  is a sole genus of vascular plant under family Selaginellaceae and order Selaginellales. The member of this order have herbaceous stems that are usually without any indication of a secondary thickening. Commonly known as spike mosses.

Source: PlantScience

Classification

  • Division: Lycophyta
  • Class: Ligulopsida
  • Order: Selaginellales
  • Family: Selaginellaceae
  • Genus: Selaginella

Occurrence

Selaginella mostly occurs in tropical region. It is worldwide in distribution. Many of them grow in damp forest. Of the species grow in temperate zones, approximately 40 species are found in United States. About 700 species found in tropical countries with moist and shady areas. Many of them grow in damp forests.

Some are xerophytic and grow on rocky cliffs or on soil that periodically becomes dry. This xerophytic species can withstand desiccation for months. Rosette like system of branches coil tightly together as the plants dry out and re-expanding when it is moistened.

  • Xerophytic: Selaginella lepidophylla (also called resurrection plant), Selaginella pilifera 
  • Epiphytic: Selaginella oregana 

Some other commercially available species are-

  • Red Selaginella – Selaginella erythropus
  • Golden Selaginella – Selaginella kraussiana
  • Peacock fern – Selaginella uncinata
  • Absorbtive fern – Selaginella braunii 

 

External Features

 

Stem

The stem is typically dichotomous often monopodial, others are climbing or sub erect. Majority are perennial and few are delicate annuals. They vary in length from few centimeters to several meters.

S. kraussiana shows a single layer of epidermal prosenchymatous fibres without any stomatal opening. Below it is the cortex of angular parenchymatous cells usually without intercellular spaces. The outer cortex cells are thickened in some, specially, xerophytic species.

The vascular cylinder in Selaginella species varies from protostelic to siphonostelic and monostelic to polystelic (with 2 to 16 separate steles). But, on the whole the stelar arrangement is simpler than in Lycopodium. S.kraussiana is distelic.

In the stele there is an exarch xylem at the centre, the metaxylem showing stalariform tracheides and a protoxylem lies towards the circumference. The xylem is surrounded by one or two layers of parenchyma which again is surrounded by a single layer of sieve tubes with sieves on lateral walls forming the phloem outside which is the peri-cycle.

 

Leaves

On the basis of the position of leaves, this genus is subdivided into two subgenera –

  1. Homoeophyllum
  2. Heterophyllum

In Homoeophyllum, the plant body found upright with spirally arranged more or less uniform leaves.

In Heterophyllum the dorsiventral and prostrate plant body is present with distinct heterophylly of leaves including majority of species. In this subgenus the leaves are two types. Two rows of large ventral leaves and two rows of small dorsal leaves.

A t.s. of the leaf  shows a simple organisation.  At the centre there is a simple vascular bundle.  The epidermis is one layer thick and the stomata are usually present on the lower epi­dermis only. The mesophyll is uniformly formed either of all spongy or all pali­sade-like elongated cells full of air spaces. Meso­phyll cells have single cup- shaped or many chloroplastids according to species.

In all cases there are several spindle-shaped, pyrenoid-like bodies at the centre of each chloroplast. The bodies may be trans- through ligule at base of leaf, formed into rudimentary starch grains. The only other Embryophyte with similar chloroplasts is the Anthocerotopsida.

Legule

On the upper surface of each leaves near the base a small flap like appendages present called Legule.

In sporophylls legule lies between the sporangium and leaf blade. it becomes prominent and functional during the growth of leaves. In  its maturity  the legule wither away .

The definite function of legule is unknown but its  predicted that it may act as water  absorbing or water secreting body.

Rhizophore

At the place of branching of stem or at the prostrate axis arises an elongated  downwardly growing, colorless, leafless cylindrical appendages called rhizophore. Each of which typically develops a tuft of adventitious roots at the free end.

This structure provides supports to weak stems.

Root

All roots borne by adult plant are adventitious in origin. In a majority of species these are formed only at the distal end of the rhizophores. but in few species like Selaginella wallichii spr adventitious root may develop anywhere along the stem.

Roots of Selaginella sp are delicate sparingly  branched, and with a small stele. Typically this is monarch in organization and has one phloem and one xylem mass. Primary roots are short lived.

 Reproduction

Vegetative reproduction takes place by

  1. Fragmentation

2. Stems or tubers

3. Bulbills

Asexual reproduction

asexual reproduction takes place by producing spores. here spores are heterosporous that means produce two types of spores.

Microsporophyll and Megasporophyll  are aggregated in strobilli and  cones at the apex of the stem or lateral branches. in Selaginella patula produces shoot with sporophyll and folliage leaves in alternate zone along the stems.

Most commonly both sporophylls borne in same strobillus

E.g.  S. kraussiana, S. helvetica 

Sporophylls may born in different srobilli.

E.g. S. gracilis

In S. rapestris  and S. selagenoides the upper portion may act as megasporangium and the lower portion may act as microsporangium.

In S.kraussiana one megaspore remains at the base and the others are microphyllus.

In S. inaquaefoiia microsporangium in one side and megasporangium in other other side.

Development of sporangium 

early development of sporangium similar up to spore mother cell. here the development of sporangium is of eusporangiate type. The sporangium arises from initial cells of the axis, however at maturity the sporangium comes to lie in the axile of the sporophyll and then it looks as if it had originated right on the  sporophyll. unlike Lycopodium sp tapetum derives from sporogenous tissues rather than jacket layer.

Right from the stage 4 onwards sporangia starts differentiating into two types of spores – microspores and megaspores.

Microspore mother cell through meiosis form microspore tetrad. in megaspore all the megaspore mother cells degenerate before meiosis except one . this one devides meiotically to form tetrad of haploid megaspores. spmetime 8,12 or 16 megaspores may occur.

In S. rapestris only one megaspore left behind.

 

Development of Male gametophtye

male gametophyte develops within the wall of microspor before it is shed from the microsporangium. the bicilliate antherozoids similar to bryophyte and lycopodium. microgametophyte is greatly reduced and is developed  entirely within the wall of microspores.

Therefore the vegetative prothalli is not essential. When the antherozoids are mature the wall of the microspore cracks open along the triradiated ridge and the antherozoids are liberated.

Development of megagametophyte 

The germination of megagametophyte within megasporangium differs in different species.

In S. kraussiana megagametophyte liberated when the first archegonium is formed.

In S. rapestris  megaspore is retained within the megasporangium untill fertilization has occured or even after embryo is developed.

The megaspore nucleus divides repeatedly forming three nuclei. the nuclei in young gametophyte lies in periferal layer of cytoplasm surrounding large central vacule.

Selaginela Kraussiana

As the nuclei increase cytoplasm becomes thicker and the vacule becomes filled completely with cytoplasm. formation of wall around the daughter nuclei occurs first at the apical part of the spore and a cushion of cellular tissues is formed.

in some species apical cushion lies in continuation with the rest of the prothallus. but in some species seperated from the rest by diafram bellow within which 3 nuclei occur.  Few archegonial initials make their appearance in the apical tissues. shortly after the spore wall breaks the apical cushion prostrudes. basal non cellular region becomes filled with large cells. these cells act as food reserve. Few rhizoids are given off from the exposed apical regions.

Fertilization may takes place while the megametophyte still with the megasporangium or after fallen or remains attached to the megasporangium for sometime the falls to the ground, where primary rhizophores develops roots into the soil and grow into the soil and start independent lifecycle.

 

The New Sporophyte

The first division of the zygote is usually transverse. The upper cell gives rise to the one or more celled suspensor and the lower cell to the embryo. The em­bryo initial divides vertically  The two resultant cells again divide vertically to form four cells one of which cuts off the stem initial from its base

Gradually, the embryo organises into sectors on one side of which is the stem between two cotyledons, on the other side is the foot (for sucking food material from the endosperm) occupying a major part of the base and the rhizophore initial on the top of it

Development of Selaginella Embryo

Life cycle of Selaginella sp

References

All images: www. biologydiscussion. com

Class lecture of Nadra Tabassum maam, Associate Professor, Department of Botany, University of Dhaka.