Pteridophytes
From the previous chapter we are aware of the
salient features of amphibious plants called Bryophytes. But there is a plant
group called Pteridophytes which are considered as first true land plants.
Further, they were the first plants to acquire vascular tissue namely xylem and
phloem, hence called vascular cryptogams. Club moss, Horsetails, quill worts, water
ferns and Tree ferns belong to this group. This chapter deals with the
characteristic features of Pteridophytes.
Pteridophytes are the vascular cryptogams and were
abundant in the Devonian period of Palaeozoic era (400 million years ago).
These plants are mostly small, herbaceous and grow well in moist, cool and
shady places where water is available. The photographs for some pteridophytes
are given in Figure 2.24.
•
Plant body is sporophyte (2n) and it is the
dominant phase. It is differentiated into root, stem and leaves.
•
Roots are adventitious.
•
Stem shows monopodial or dichotomous branching.
•
Leaves may be microphyllous or megaphyllous.
•
Stele is protostele but in some forms siphonostele
is present (Marsilea)
•
Tracheids are the major water conducting elements
but in Selaginella vessels are found.
•
Sporangia, spore bearing bag like structures are
borne on special leaves called sporophyll. The sporophylls gets organized to
form cone or strobilus. Example: Selaginella,
Equisetum .
•
They may be homosporous
(produce one type of spores-Lycopodium)
or Heterosporous (produce two types
of spores-Selaginella).
Heterospory is the origin for seed habit.
•
Development of sporangia may be eusporangiate (development of sporangium from group of initials) or leptosporangiate (development of
sporangium from single initial).
•
Spore mother cells undergo meiosis and produce
spores (n).
•
Spore germinates to produce haploid, multicellular
green, cordate shaped independent gametophytes called prothallus.
•
Fragmentation, Resting buds, root tubers and
adventitious buds help in Vegetative reproduction.
•
Sexual reproduction is Oogamous. Sex organs, namely
antheridium and archegonium are produced on the prothallus.
•
Antheridium produces spirally coiled and
multiflagellate antherozoids.
•
Archegonium is flask shaped with broad venter and
elongated narrow neck. The venter possesses egg or ovum and neck contain neck
canal cells.
• Water is essential for fertilization.
After fertilization a diploid zygote is formed and undergoes mitotic division to form embryo.
• Pteridophytes show apogamy and apospory.
Reimer (1954) proposed a classification for
Pteridophytes. In this classification, the Pteridophytes are divided into five subdivisions.
1. Psilophytopsida
2. Psilotopsida
3. 3.
Lycopsida
4. Sphenopsida
5.
Pteropsida.
6.
There are 19 orders and
48 families in the classification.
The Economic importance of Pteridophyte is given in
Table 2.4
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
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).
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.
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.
The stem may be erect, dichotomously branched or
prostrate with lateral branching. The prostrate stem is dorsiventral.
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).
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).
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.
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.
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.
Selaginella shows both vegetative and asexual modes of reproduction.
Selaginella
reproduces
vegetatively by fragmentation, bulbil
formation, tuber formation and resting buds.
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).
Division – Pteropsida
Class - Leptosporangiopsida
Order – Filicales
Family – Polypodiaceae
Genus – Adiantum
Adiantum is commonly known as ‘Maiden hair fern’ or
‘Walking fern’. They are distributed in the tropical and temperate regions of
the world. Some of the Indian species include Adiantum capillus-veneris, Adiantum
pedatum, Adiantum caudatum and Adiantum
venustum. The sporophyte is differentiated into rhizome, roots and leaves
Figure 2.31.
The rhizome is a perennial, subterranean
dichotomously branched structure and is creeping in A. capillus-veneris or may be erect as in A. caudatum. It is covered with persistent leaf bases and hairy
outgrowths called ramenta.
The roots are adventitious and arise from the
rhizome.
The epidermis is the outermost layer and bears
unicellular root hairs. The cortex is divided into outer wide parenchymatous
and inner narrow sclerenchymatous layer. The stele is simple and possesses a
central core of xylem in diarch condition with phloem on either side of it
(Figure 2.32).
The rhizome in transverse section shows a single
layered epidermis covered by cuticle. Some epidermal cells bear multicellular
hairs. The Epidermis is followed by two to three layered hypodermis made up of
sclerenchyma tissue. A parenchymatous ground tissue is present. The young
rhizomes have amphiphloic siphonostele. The older rhizomes have solenostele or dictyostele
(Figure 2.33).
The petiole in T.S. shows a single layered
epidermis with thick cuticle. Epidermis is followed by a sclerenchymatous
hypodermis which provides mechanical support. There is an extensive parenchymatous
ground tissue. The central region possesses a single large horse shoe shaped
stele. Xylem forms central core surrounded by phloem (Figure 2.34).
The Pinnule shows upper and lower epidermis. The
cells contain chloroplasts. Stomata are confined to lower epidermis. The
mesophyll is not differentiated into palisade and spongy parenchyma. The
vascular bundle is surrounded by sclerenchymatous bundle sheath.
Adiantum is
homosporous. The reproduction takes place
by the production of spores. The spores are produced in sporangia. A group of
sporangia forms sori. The sori are marginal but the reflex margins of the pinna
form a protective membranous structure called false indusium (Figure
2.35). The development of sporangium
is of leptosporangiate type.
The sorus does not show any definite sequence hence
fall under mixed type.
A mature sporangium bears a multicellular stalk and
a spherical or elliptical single layered structure called capsule. The capsule contains haploid spores. The wall of the
capsule is differentiated into thick walled annulus and thin walled stomium. On maturity the sporangium
bursts and spores are released. The spores germinate and undergo repeated
division to produce a prothallus.
The prothallus is flat, green and heart shaped. It is monoecious and represents
the gametophytic phase. Sex organs called antheridia
and archegonia develop on the
prothallus. Antheridia release multiflagellate antherozoids which swim in water
and reach the egg of the
archegonium to accomplish fertilization. The
fertilization results in zygote(2n) and it represents the first cell of
sporophytic generation. The zygote develops into embryo which further
differentiates into sporophyte. Thus Adiantum
shows alternation of generation (Figure 2.36).
The term stele refers to the central cylinder of
vascular tissues consisting of xylem, phloem, pericycle and sometimes medullary
rays with pith (Figure 2.37).
There are two types of steles
1.
Protostele
2.
Siphonostele
In protostele phloem surrounds xylem. The type
includes Haplostele, Actinostele, Plectostele, and Mixed protostele.
(i) Haplostele: Xylem surrounded by phloem is known as haplostele.
Example: Selaginella.
(ii) Actinostele: Star shaped xylem core is surrounded by phloem is known
as actinostele. Example: Lycopodium serratum.
(iii) Plectostele: Xylem plates alternates with phloem plates. Example: Lycopodium clavatum.
(iv) Mixed prototostele: Xylem
groups uniformly scattered in the
phloem. Example: Lycopodium cernuum.
2.
Siphonostele:
In siphonostele xylem is surrounded by phloem with
pith at the centre. It includes Ectophloic siphonostele, Amphiphloic siphonostele,
Solenostele,
(i)
Ectophloic
siphonostele: The phloem is
restricted only on the external side of the xylem. Pith is in centre. Example: Osmunda.
(ii)
Amphiphloic
siphonostele: The phloem
is present on both the sides of xylem. The pith is in the centre. Example: Marsilea.
(iii) Solenostele: The stele is perforated at a place or places corresponding the
origin of the leaf trace.
(a)
Ectophloic solenostele – Pith is in the centre and the xylem is surrounded
by phloem Example Osmunda.
(b)
Amphiphloic
solenostele – Pith is in
the centre and the phloem is present on both sides of the xylem. Example: Adiantum pedatum.
(c)
Dictyostele
– The
stele is separated into several
vascular strands and each one is called meristele. Example: Adiantum capillus-veneris.
(iv) Eustele: The stele is split into
distinct collateral vascular bundles
around the pith. Example: Dicot stem.
(v) Atactostele: The stele is split into
distinct collateral vascular bundles and are scattered in the ground tissue
Example: Monocot stem.
(vi) Polycyclicstele: The vascular tissues are present in the form of two or more
concentric cylinders. Example: Pteridium.
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