Selaginella - Class 11 PDF Download

Selaginella

Systematic Position

Note : Common name –  [Little club moss or spike moss]

Habitat & Habit

1. Most species are found in tropical rainforest
2. The common Indian species are Selaginella kraussina, S. pallidissima and S.oregana etc.
3. S. pilifera, S. rupesteris and S. lepidophylla are the common xerophytic species and are called resurrection plant

Note : Xerophytic species of selaginella show cespitose habit. They become ball like during dry season and again become green and fresh when moisture is available. So these are called resurrection plants. Selaginella bryopteris is known as "Sanjivani Plant".

External Morphology

1. Main plant body is sporophyte.
2. Plant body is distinguished into three parts Root, Stem and Leaves. Rhizophore also found in some species.

ROOT  :  Primary root are ephemeral and the adult plant has adventitious roots. Root hairs are unicellular.

• These are branched with root hairs.

Stem: Stem is green and cylindrical. Stem is prostrate and laterally branched (in some species) and erect and dichotomously branched.

• Adventitious roots arise from the stem and stem is covered by leaves.
• A structure arise from the dichotomous [bifurcation] point of stem which is colourless without chlorophyll, cylindrical, rootcap less and elongated is called Rhizophore 

Morphological Nature Of Rhizophore  :

• Rhizophore arises from the dichotomous point of stem. It is Non-chlorophyllous colourless, long cylindrical and thread like structure. Rhizophore term given by Nageli.
• It grows toward the soil and it is bifurcated. It provides support to the plant and help in absorption of water and minerals.
• Morphological nature of the Rhizophore of selaginella is still in question. The following three views about its possible nature are–

(a)  Rhizophore is  a Root cap less root  - According to Van Tieghem, Harvey Gibson "rhizophore is a root cap less root" because-

(1) It is positively geotropic in nature

(2) It is leafless

(3)  Rhizophore is monosteleic like root.

(b)  Rhizophore is  leafless shoot – Pfeffer , Treub and Cusick, Bruchmann etc regards rhizophore a leafless shoot because-

(1) Absence of root cap

(2) Absence of root hair

(3) It arises from bifurcation point of stem

(4)  It is exogenous in origin

(c)  Rhizophore is a newly formed organ – According to Goebel and Bower the rhizophore is neither shoot nor a root but organ sui-generies.

Leaves  : The leaves of selaginella are microphyllous, sessile and simple. Their shape varies from ovate to lanceolate. The leaves are mostly Amphistomatic (Exception S. martensii-Hypostomatic)

• The vegetative leaf, as well as sporophyll, has a small, membranous projection on its adaxial (upper) surface, close to the base. This projection is known as ligule.
• The basal part of the ligule has a distinct hemispherical foot-like structure, called glossopodium. The ligule is embedded at the base of the leaf in a pit-like structure known as ligular pit.
• Leaves are dimorphic or Heterophyllum. The megaphylls are develop from ventral/Abaxial/ lower surface of the stem and microphylls develop from dorsal/adaxial/upper surface of the stem.
• S.spinulosa, S.rupestris, S.pygmea and S.oregana have monomorphic leaves.

Function of ligule :

• According to some scientists ligules are associated with water absorption and secretion and thus prevent dessication of the shoot.
• Ligules are concerned with upward movement of inorganic solutes.

Special Point : 

1. A distinctive feature of selaginella stem is the presence of radially elongated endodermal cells, called trabeculae. Between these cells large intercellular spaces are present.

2. Trabeculae are absent in the stem of xerophytic species of selaginella.

Reproduction - The sporophyte of Selaginella reproduces vegetatively and by spores.

1. Vegetative Reproduction : It is rare in selaginella. Its occurs by the following method.

1. Fragmentation     
2. Resting Bud                              
3. Tuber                            
4. Apogamy

Fragmentation :    It occurs during moist condition.

2. By resting Bud : Terminal leaves overlapped to each other and become fleshy.

• They form resting bud. They are formed for vegetative reproduction. eg., Selaginella chrysocaulos.
• By Tuber : Subterranean branches of S.chrysorhizos enters in substratum. Their terminal and swollen end form tuber. During favourable condition they form new plant.

4. Apogamy :  Sometime gametophyte directly develops in sporophyte. These sporophyte are haploid.

(2)  Reproduction by spores

• Selaginella is a heterosporous Plant. It produces two types of spores – megaspores and microspores.

(i) Strobilus or cone - 

• In most of the species of Selaginella sporophylls are aggregated at the apex of the main stem or lateral branch and forms compact cones, called strobili. (Singular - strobilus)
• The sporophylls are spirally arranged around a central axis.
• The sporangia are borne singly in the axils of sporophylls.
• The sporangia are strictly dimorphic.
• There are many microspores in a microsporangium but a megasporangium usually has 4 megaspores.
• The megaspores form female gametophytes on germination and the microspores give rise to male gametophytes.

(ii) Distribution of micro and megasporangia in strobilus

• In most of the species of Selaginella both micro and megasporangia are found within the same strobilus.

Development of sporangium   (Eusporangiate type)

1. Microsporogenesis : Each microspore mother cell divide meiotically to form haploid tetrad sproes in large numbers [128 or 256]

(b) Megasporogenesis: 

• All the cells of megasporangium are degenerate except only one sporocyte.
• This functional cells is called megaspore mother cell. This cell divide meiotically to form four megaspores.
• S. monospora &  S. rupestris bears only single megaspore in megasporangium.

Microspores And Development Of Male Gametophyte:

1.   Microspore : 

• The microspores are small spherical structures, ranging 0.015-0.06 mm in diameter. A microspore is surrounded by a thick ornamented exine and a relatively thin intine.
• The spore has a single haploid nucleus and granular cytoplasm, rich in fatty substances (about 35 %) and chlorophyll absent.

     
  2.   Development of male gametophyte : 

• The microspore germinate inside the microsporangium and shed from the microsporangium at
  13-celled stage.
• The first division of the microspore is asymmetrical and as a result a small lenticular prothallial cell and a large antheridial initial is established. The prothallial cell does not divide further.
• The first division of the antheridial initial is nearly at right angles to the prothallial cell. It result in the formation of two antheridial cells of almost equal size. Both these cells divide by a vertical wall (at right angles to the first vertical division). Thus, at this stage the gametophyte consists of five cells (four cells derived from the antheridial initial and a prothallial cell). The two basal cells, derived from the antheridial initial, do not divide further, whereas the upper two daughter cells divide repeatedly and form ten cells. At this stage the gametophytes has 13 cells. Of these, four central cells function as primary androgonial cells and eight peripheral cells function as jacket cells.
• The male gametophyte shed from the microsporangium at  13-celled stage. (1- prothallial cell, 4- primary androgonial cells and 8- jacket cells)
• The four central primary androgonial cells of the male gametophyte divide repeatedly forming a mass of 128-256 antherozoid mother cells or androcytes. Each androcyte by metamorphosis converts into a spindle-shaped/comma-shaped biflagellate antherozoid.
• With the formation of antherozoids, the jacket cells decompose and form a mucilaginous substance. The antherozoids float in this substance. Until this stage the male gametophyte is completely enclosed within the wall of the microspore. The gametophyte is not set free and is dependent on the parent sporophyte for nutrition.

Note : 

1. Unlike other pteridophytes, vegetative prothallus are not formed in selaginella.

2. The antherozoids of Selaginella are the smallest among the vascular plant.

3. Development of gametophyte from spore start before dehiscence of spore, thus it is known as Precocious or In-situ germination.

Megaspore and development of female gametophyte

• Their diameter varies from 0.15 to 0.5 mm
• The megaspores are arranged in tetrahedral tetrads.
• The wall of the megaspore is differentiated into three distinct layer – the outer exospore, the middle mesospore and the inner endospore.
• The megaspore has a single haploid nucleus, surrounded by granular cytoplasm rich in fatty substances (about 48%)
• The development of female gametophyte starts even when the megaspore remain present in the megasporangium.
• By repeated free nuclear division followed by cell wall formation female gametophyte is differentiated into upper region called female prothallus and lower region called storage region which are separated by diaphragm.
• A functional cell of the female prothallus acts as archegonial initial.

Note : The mature archegonium of Selaginella has a long neck (consisting of eight cells, in two tiers of four cell each), a neck canal cell, a venter canal cell and an egg.     

Fertilization :

• Fertilization usually takes after the female gametophyte has fallen in the soil but in some species (e.g. S.rupestris) it may occur while the female gametophyte is still within mega sporangium.
• Water is essential for fertilization. Just before the fertilization the neck and ventral canal cell disorganize and to form a mucilage which contain some chemotactic substances.
• Many biflagellated antherozoids enter into the neck of archegonium by chemotactic movement and one antherozoid fuse with the egg, resulting in the formation of zygote.

Note : In few species like S. rupestris archegonia are fertilized when enclosed in the megasporangium which is attached with spike hence approach towards the seed habit of Angiosperm. Heterospory is a pre-requisite condition for the seed habit.

Embryo Developement

1. The diploid zygote is the mother cell of the sporophytic generation.
2. It divides transversely, establishing an epibasal (upper) suspensor cell and a hypobasal (lower) embryonic cell. As development proceeds, the suspensor cell repeatedly divides to form a suspensor, which pushes the developing embryo deep into the female gametophyte. 
3. The rest of the embryo develops from the embryonic cell. It divides by two vertical wall at right angles to each other and thus a four-celled embryo is formed. In these, one cell divides by an oblique vertical wall and thus an apical cell with three cutting faces is established. This eventually functions as the apical cell of the embryonic shoot.

The remaining three cells of the 4-celled embryo and the sister cell of the apical cell divide transversely to form two tiers of four cells each.

1. The cells of both the tiers divide irregularly, forming a multicellular embryo. Usually the cells of the lower tier divide more rapidly than the upper tier.
2. The derivatives of the lower tier form the foot. The foot acts as a haustorial organ; its main function is to absorb nutrition for the developing sporophyte from the female gametophyte.
3. Two cells of upper tier acts as apical cell of cotyledon and forms two ligulated cotyledon.
4. The part of the embryo posterior to cotyledons develops into hypocotyledonary part of the stem. The stem grows with the help of the apical cell of the embryo.
5. After the formation of cotyledons and stem, the apical cell of the root differentiates on the lateral surface of the foot. The derivatives of this cell develop into a root-like structure, called rhizophore. Roots, infact, develop at the apex of the rhizophore.
6. In early stages of development the young sporophyte is attached to the megaspore and derives its food from the female gametophyte with the help of its foot. But after the establishment of root and stem, the sporophyte becomes independent.

Life Cycle : Diplo - Haplontic