SELAGINELLA
Selaginallales
CONTENTS
• CLASSIFICATION
• HABIT AND HABITAT
• EXTERNAL MORPHOLOGY
• INTERNAL MORPHOLOGY
CLASSIFICATION
• DIVISION: Lycophyta
• CLASS:
Ligulopsida
• ORDER:
Selaginellales
• FAMILY:
Selaginellaceae
• GENUS:
Selaginella
• SPECIES: S.kraussiana (very common)
S.monospora
S.picta
HABIT AND HABITAT
• Selaginella is the only living genus of the order
Selaginellales.
• Commonly known as ‘spike moss’ or ‘small
club moss’.
• It is a large genus comprising of about 700
species distributed all over the world.
• Abundantly it is found growing in tropical rain
forests.
• Habit: grows in moist and shady places.
• Epiphytic: S.oregana
• Xerophytic: S.lepidophylla
HABIT AND HABITAT
• A few xerophytic species of Selaginella e.g., S. lepidophylla and S. pilifera are called
resurrection plants. They curl and become ball like when dry and again become green and
fresh when moisture is available.
• About 70 species have been reported from India.
• They are mainly found growing in eastern as well as Western Himalayas and the hills of South
India.
• Some of the common Indian species are S. repanda, S. biformis, S. denticulata, S. monospora,
S. semicordata, S. adunca etc.
• S. kraussiana is cultivated in green house.
EXTERNAL MORPHOLOGY
• The sporophyte is an evergreen, delicate herb.
• Its size varies greatly from species to species i.e., from a few cm. to 20 meters.
• Plants may be erect or prostrate depending upon the sub-genus.
The Sporophyte
• The Sporophytic plant body is distinctly differentiated into following structures:
• STEM
• LEAVES
• LIGULES: a ligule is present at the base of each leaf and sporophyll.
• RHIZOPHORE: a leafless structure where from roots arises. It is present in some species.
• ROOTS
1. Stem
• It is usually profusely branched, delicate and
evergreen.
• The growing apex of the stem consists of either
meristematic tissue or a single apical cell.
• In the sub-genus homoeophyllum the stem is erect
and somewhat cylindrical and in the sub-genus
heterophyllum it is prostrate with stout erect
branches and is somewhat dorsiventral.
2. Leaves
• They are usually small, simple and lanceolate with a pointed apex. Each leaf is provided with a single
unbranched midrib. In the sub-genus homoeophyllum all the leaves are of same size and are spirally
arranged forming a dense covering.
• In the sub-genus heterophyllum the leaves are dimorphic i.e., of two size (small and big) and are
arranged in pairs. Small leaves are present on the dorsal side of the stem and bigger ones on the ventral
side of the stem (Fig. 1 B). The bigger leaves alternate with bigger ones and smaller leaves alternate with
smaller ones.
• Usually the leaves near the apical portion of the branch, bear sporangia (micro-or mega) and are called as
sporophylls (micro-or mega) respectively. The sporophylls are usually aggregated into a condense
structure which is known as strobilus.
3. Ligules
• On the adaxial side of the leaf, near the base is
present a small membranous out-growth known
as ligule.
• It is embedded at the base of a leaf in a pit like
structure known as ligule pit.
• It may be tongue shaped (e.g., S. vogelii), fan
shaped (e.g., S. martensii), fringed (S. cuspidate
), or lobed (e.g., S. caulescens).
• It is more than one cell in thickness except at the
apex.
• The structure of the ligule can be differentiated
into two parts, glossopodium and the body of
the ligule.
3. Ligules
• Glossopodium:
• It is the basal hemispherical part made up of large thin walled cells.
• It is surrounded by a glossopodial sheath.
• Body of the ligule:
• Above the glossopodium is the body of ligule.
• It is made up of many large and small cells.
• The function of the ligule is not well known.
• It may be a water secreting or water absorbing or protective organ.
• According to Earner (1936) the ligule is perhaps a vestigial organ.
4. Rhizophore
• In many species of Selaginella, peculiar leafless, prop-like cylindrical, structures, originate from the
stem at the point of branching.
• These grow downwards into the surface and form many adventitious roots at their free ends. They are
known as rhizophores.
• In few species the rhizophore is present e.g., S. krussiana while in others it is absent e.g., S. cuspidata.
• It differs from root in having no root cap and from stem in having no leaves.
• The morphological nature of rhizophore is controversial because of its unusual position and structure.
• It has been interpretated by various plant scientists as root, stem or an organ sui generis (i.e., an organ,
neither a stem nor a root).
4. Rhizophore
• I. Similarities with Root
• Rhizophores are positively geotropic in nature.
• It does not bear leaves.
• Monarch xylem like that of root.
• Presence of root cap in some species, e.g., S. densa, S. kraussiana, S. martansii, S. wallacei
• Transport of auxin in rhizophore is acropetal which is similar to root
• II. Similarities with Stem
• Exogenous in origin like stem.
• Absence of root caps and root hairs.
• Originate due to the activities of meristems which are present between the two branches of the stem. This meristem
has been termed as angle meristem which is basically an embryonic shoot.
• Production of roots endogenously from the tip.
• Under experimental conditions the rhizophore can be transformed into a leafy short
4. Rhizophore
• Bower (1908, 1935) and Goebel (1905) suggested that rhizophore is neither a root nor a stem, but an
organ sui generis.
• According to Schoute (1938) it is a specialised stem behaves like root.
• However, recent biochemical studies of protein from stem, leaf, root and rhizophore revealed that the
polypeptides of the rhizophore more closely resemble those of the stem rather than subterranean roots.
• The above-mentioned features are not for a typical root, moreover, they produce roots endogenously.
• Therefore, these outgrowths are called rhizophores (Gr. rhiza = root; phora = bearer).
5.Roots
• They originate either from the tips of rhizophores or directly from the stem or from the swollen
base of hypocotyl.
• Their origin is endogenous.
• They are usually dichotomously branched structures.
• The roots are provided with root caps and root hairs.
INTERNAL MORPHOLOGY (STEM)
1) STEM: A Transverse section (T.S.) of the stem of Selaginella is somewhat circular in outline and shows
the following structures:
• Epidermis –
• It is the outer most covering layer comprising of a single cell in thickness.
• The cells of the epidermis are without hairs and stomata.
• The epidermis is surrounded on all sides by a thick coating of cuticle.
• Cortex –
• Inner to the epidermis is present a well-defined zone of cortex.
• The cortex may or may not be differentiated into inner and outer cortex.
• In case of S. selaginoides, the whole of the cortex is made up of parenchymatous cells while in S. kraussiana, it is differentiated
into sclerenchymatous outer cortex and parenchymatous inner cortex.
• The parenchymatous cortex is usually made up of angular cells i.e., without intercellular spaces but in some cases the cells are
rounded and provided with a few inter-cellular spaces.
INTERNAL MORPHOLOGY (STEM)
• Stele:
• Their stele is from monostelic to polystelic condition.
• Each stele is protostelic in nature.
• The metaxylem forms the solid central core. The protoxylem groups on the periphery. Xylem is
usually made of tracheids. Vessels are completely absent.
• Xylem is surrounded on all sides by phloem which consists of sieve cells and phloem parenchyma.
Companion cells are absent in phloem
• Outside the phloem is the pericycle.
• It is composed of single layer of parenchymatous cells.
• The stele is separated from the cortex by a wide, air space.
• These spaces have long radiating cells called trabeculae.
INTERNAL
MORPHOLOGY
(STEM)
INTERNAL MORPHOLOGY (ROOT)
2) ROOT: A T.S. of the root is somewhat circular in outline and shows the following internal structures:
• Epidermis:
• It is the outermost covering layer and is only one cell in thickness.
• The cells are large and the unicellular root hairs arise from them.
• Cortex:
• Just below the epidermis is present a wide zone of cortex.
• The cortex may be either wholly made up of thin walled parenchymatous cells or there may be sclerenchymatous outer cortex
(hypodermis), 3 to 5 celled in thickness and parenchymatous inner cortex.
• Stele:
• It is a typical protostele. The xylem is exarch and monarch i.e., there is only one protoxylem group situated at the periphery.
• Xylem is surrounded by phloem on all sides. The structure of xylem and phloem elements is similar to that of stem.
• Phloem is surrounded by one to three layered pricycle. It is made up of parenchymatous cells.
• Endodermis is usually not well defined but in some species as for example, S. densa, it is a distinct structure and only one cell
in thickness.
INTERNAL
MORPHOLOGY
(ROOT)
INTERNAL MORPHOLOGY (RHIZOPHORE)
3) RHIZOPHORE:
The internal structure of rhizophore is
almost similar to that of root. It is also
circular in outline.
INTERNAL MORPHOLOGY (LEAF)
• A T.S. of a leaf shows two epidermal layers, mesophyll tissue, stele and stomata.
• The cells of upper and lower epidermal layers may be similar in most of the species (e.g., S. rupestris).
• However, they may be somewhat different in some species (e.g., S. martensii) where the upper epidermis
consists of conical cells, but the cells of the lower epidermis are smaller.
• The mesophyll tissue may be differentiated into a distinct palisade and spongy parenchyma layers (e.g.,
S. lyalli, S. concinna) or the entire mesophyll may be parenchyma cells. A mesophyll cell contains 1-8
cup-shaped chloroplasts with many pyrenoid-like cells.
• The stomata are present generally on the abaxial (lower) surface, although in certain species they are
present on both the surfaces (amphistomatic).
• A single vascular bundle composed of central xylem surrounded by phloem is present at the centre. The
bundle is bounded by a distinct bundle sheath.
INTERNAL MORPHOLOGY (LEAF)
VEGETATIVE REPRODUCTION
• Fragmentation:
• Under humid conditions in S. rupestris, trailing branches of the stem develop adventitious branches.
• These branches later disjoin from the parent plant and develop into separate individual plants.
• Tubers:
• These appear towards the end of the growing season.
• The tubers may be aerial, developing at the apical end of aerial branches (e.g., S. chrysocaulos) or
subterranean (e.g., S. chrysorrhizos).
• Under favourable conditions tubers germinate into a new plant.
• Resting buds:
•
•
•
•
These are the compact structures which develop at the apical end of some aerial branches.
The leaves in this region are closely arranged and overlap the growing points.
These resting buds are capable to pass on the unfavourable conditions.
Under favourable conditions these buds give off rhizophore that bear roots at their tips.
VEGETATIVE REPRODUCTION
SEXUAL REPRODUCTION
• Selaginella is a sporophytic plant (2x) and
reproduces sexually.
• The plants are heterosporous i.e., produce
two different types of spores—megaspores
and microspores.
• These spores are produced in megasporangia
and microsporangia, respectively which, in
turn, are produced on fertile leaves known as
megasporophylls and microsporophylls
respectively.
• Usually both these structures are grouped
together to form a compact structure known
as strobilus which is usually a terminal
structure.
STROBILUS
• Numerous haploid spores are produced in the sporangium. The
sporangium are located in the sporophylls and the sporophylls are
compactly arranged to form cones or strobili.
• Generally strobili occur terminally on side branches.
• Megasporangia are present in the basal portion and the microsporangia
are present in the upper part of the cone.
• Each microsporangium contains several microspores. But there are only
four megaspores in each sporangium. The mature spores are pyramidal in
shape.
• The wall of sporangium consists of three layers. The inner most layer is
tapetum. They provide nourishment to the developing spores. A slit is
produced in mature sporangia. The spores come out of this slit. The
spores germinate to develop gametophytes.
• Microspore give rise to male gametophytes and the megaspores produces
female gametophytes.
SPORANGIUM
• The mature sporangia are stalked
with two- layered jacket.
• The microsporangia are slightly
elongated and reddish to bright
orange in colour.
• Megasporangia are larger than
microsporangia and are frequently
lobed. The megasporangia are
whitish-yellow or light orange in
colour.
DEVELOPMENT OF SPORANGIUM
• Initially the development of micro- and megasporangia are similar. The sporangial development is of
eusporangiate type i.e., the sporangium develops from a group of initial cells.
• The site of sporangial initiation (micro- or mega-sporangium) is in superficial cells of the axis, directly
above the sporophyll, or in cells near the base of the sporophyll on the adaxial (upper) side.
SPOROGONAIL
INITIAL
Inner
archesporial
Initials
Outer jacket
initials
Outer layer
Tapetum
Inner layer
Sporogenous
tissue (Spore
mother cells)
SPOROGENOUS
TISSUE
Tetrads
Microspores
Only one
megaspore mother
cell becomes
functional
Four megaspores
DIFFERENTIATION OF MICRO AND
MEGASPORANGIUM
• Further development of sporogenous tissue gives rise to micro and megaspores.
• In micro- sporangium, about 80-90% of the sporogenous tissue undergoes meiosis and forms tetrads of haploid
microspores. The microspores are more or less tetrahedral in shape.
• They are quite small in size (0.015 to 0.06 mm in diam.). The wall of the spore is divisible into outer thick and
ornamented exine and inner thin intine.
• In a potential megasporangium, only one spore mother cell becomes functional. The remaining non-functional
megaspore mother cells develop large vacuoles and accumulate starch, while functional megaspore mother cell
retains a dense cytoplasm which is rich in RNA and lacks starch.
• The functional megaspore mother cell undergoes meiosis (reductional division) forming four megaspores. All
the nonfunctional mother cells ultimately degenerate.
• All the four megaspores develop from a megaspore mother cell may not be functional.
• The resulting megaspores soon develop a thick-layered cell wall. The outer layer is thick and ornamented with
spines or ridges known as exine (exospore). The inner layer is thin and termed as intine (endospore). In some
species a third layer (mesospore) forms between exine and intine. The megaspores are much larger (diam. 1.5 to
5 mm) than the microspores.
MALE GAMETOPHYTE
• The haploid spores germinate to form the endosporic gametophytes. The development of microspores
and megaspores generally starts while they are still inside their respective sporangia. Therefore, the
spores are shed at multicellular stage.
• Male gametophyte is shed at this 13- celled stage (1 prothallial cell, 8 jacket cells and 4 androgonial
cells).
MALE GAMETOPHYTE
• After shedding, the four primary androgonial cells undergo several divisions forming 128 or 256
androcytes.
• Each androcyte gets metamorphosed into a biflagellate sperm (antherozoid) which on disintegration of
the jacket cells and rupture of the spore wall along the triradiate ridge are liberated (as free-swimming
sperm/antherozoid) (Fig. 7.54).
• The antherozoids (sperms) are free-swimming type, very narrow and about 15-50 pm in diameter at about
the middle of the sperm/antherozoid. Each flagellum is about 30 pm in length. The sperms of Selaginella
are the smallest among vascular plants.
• Since the whole process of development of male gametophyte takes place inside the microspore wall, the
development is endosporic (where gametophyte develops inside the spore wall) and the gametophyte is
an extremely reduced structure.
FEMALE GAMETOPHYTE
• Like the male gametophyte, the megagametophyte (female gametophyte) development of Selaginella begins while
the megaspores are still inside the megasporangium.
• A conspicuous vacuole develops within the cytoplasm of the megaspore at a very early stage. Thus, the megaspore
nucleus undergoes repeated nuclear divisions without cell wall formation (free nuclear division). As a result a thin
layer of multinucleate cytoplasm is developed around the large vacuole.
• The formation of cell walls around each nucleus starts initially at the apical end beneath the triradiate ridge and an
apical patch of cells, two to three layered thick, is formed.
• Ultimately the vacuole is obliterated by the formation of cells. The stage at which the megaspore is shed from the
megasporangium varies from species to species.
• The female gametophyte increases in size and exerts pressure on the megaspore wall.
• This results in the splitting open the megaspore wall along the triradiate ridge. Tufts of rhizoids may develop from
the exposed gametophytic tissue which play an important role in absorption of water and nutrients, also in
anchorage.
• However, in S. rupestris, the megaspores are not shed (retention of megaspore in megasporangium) and the
development of female gametophyte and fertilisation take place in situ and the young sporophyte can be seen
developing on the parent plant.
DEVELOPMENT OF FEMALE
GAMETOPHYTE
DEVELOPMENT
OF
ARCHEGONIUM
DEVELOPMENT OF ARCHEGONIUM
• Archegonia originate in the apical region of the female gametophyte. Each archegonium develops from a
single superficial cell (archegonial initial) which divides to form a primary cover cell and a central
cell.
• The primary cover cell follows two vertical divisions at right angle to each other to form four neck
initials which again divide transversely to form eight neck cells, arranged in two tiers. The central cell
divides periclinally to form an outer primary neck canal cell and an inner primary venter cell.
• The primary neck cell does not divide further so that a neck canal cell is formed, while the primary venter
cell divides transversely into a ventral canal cell and an egg.
• Thus, a single archegonium consists of eight neck cells arranged in two rows of four cells each, one
neck canal cell, one ventral canal cell and an egg.
• The venter, along with the inner tier of neck, lies embedded in the gametophytic tissue, while the terminal
neck cells extend above the surface of the gametophytic tissue.
FERTILIZATION
• In majority of the species, fertilisation takes place after the megasporangium gets settled on the
substratum.
• In some cases (e.g., S. rupestris), fertilisation occurs while the female gametophyte is still within the
sporangium.
• Biflagellate sperms (haploid) are liberated, then they swim to the archegonia through a thin film of water
and fertilise the egg (haploid) to form diploid zygote.
EMBRYO
• After fertilisation, the diploid sporophytic generation (i.e. zygote) is established. The first division of the
zygote is generally transverse.
• The upper cell (epibasal) develops into one- or several-celled suspensor (that are toward the archegonial
neck) and the lower cell (hypobasal) to the embryonic cell.
• The embryo is endoscopic (embryonic cell directed toward the base of the archegonium) in nature.
• The embryonic cell divides by two vertical walls at right angles to each other resulting into a four-celled
embryo.
• The first pair of leaves are developed laterally on two sides of the shoot apex.
• Eventually a foot is formed on the lower side of the embryonic tissue and the primary root is developed
between the suspensor and foot.
• The young sporophyte emerges from the gametophytic tissue through continued growth of the shoot and
root.
DEVELOPMENT OF EMBRYO
LIFE CYCLE