Introduction to botany
Pteridophytes structure and reproduction
Introduction A fern, or Pteridophyte, is any one of a group of plants classified in the Division Pteridophyta,
formerly known as Filicophyta.Ferns are seedless vascular plants and reproduces by spores and by alternating generations of
separate spore producing plants (sporophytes) and gamete producing plants (gametophytes).
.A fern is a vascular plant that
differs from the more primitive lycophytes in having true leaves (megaphylls) and from the more advanced seed plants
(gymnosperms and angiosperms) in lacking seeds, and instead reproducing with spores. It comprises one of the largest divisions
in the kingdom plantae with over 10,000 different species.
Evolution and ClassificationFerns first appear in the
fossil record in the early-Carboniferous period. By the Triassic, the first evidence of ferns related to several modern
families appeared. The "great fern radiation" occurred in the late-Cretaceous, when many modern families of ferns first
One problem with fern classification is the problem of cryptic species. Cryptic species are those which are morphologically
similar to another species, but which differ genetically in ways that prevent fertile interbreeding.Ferns have traditionally
been grouped in the Class Filices, but modern classifications assign them their own division in the plant kingdom, called
According to Bold et al.
(1987) and Lelinger (1985), the ferns have been a problem in phylogenetics for some time. The classical relationships of the
groups of ferns can be seen in Pearson (1995) and Rothwell (1999), both of which are similar to the view of Bold et al. (1987).
Pryer et al. (2001), however, through molecular phylogenetic analysis indicate that the ferns, if considered as a monophyletic
group, must include the psilophytes and horsetails. That was the step taken by Smith et al. (2006) when they offered a revised
Linnaean taxonomy of extant ferns.
Habitats:- Pteridophytes occur in both hills as well as in plains in cold, moist
and shady places.
They also occur in humid and tropical climates and usually grow
on soil, rocks, in ponds and as epiphytes on other plants.
Many ferns rely on the associations of the mycorrhizal fungi. The ferns grown in specific pH ranges with higher levels of acid
such as the bulblet bladder fern are only found on limestone.
General features of Pteridophytes :-
- The plant body of pteridophytes varies in two distinct phases
during their life cycle:- The Gametophytic Phase and the Sporophytic
- The diploid sporophyte is the dominant phase in the life cycle and the sporophyte is independent of
The Sporophytic Plant Body:-
- The sporophyte is generally herbaceous and is differentiated into true
roots (adventitious), stem and leaves. The leaves may be small
microphyllous or large macrophyllous (fronds).
- They are chlorophyllous and Autotrophic.
- All vegetative parts possess vascular tissues organized into steles or
vascular bundles. So, pteridophytes are are first tracheophytes in
evolution of plant kingdom.
- The sporophyte performs vegetative reproduction and asexual
- Vegetative reproduction takes place by vegetative buds that develop
on the rhizome or by fragmentation of rhizome.
- Asexual reproduction takes place by means of spores produced inside
- The sporangia are borne on lower surface or in axils of fertile leaves
- The sporangia are borne singly or in groups called Sori.
- Plants may be Homosporous i.e., they produce only one type of
spores or may be Heterosporous i.e., produce two different types of
spores-Smaller Microspores and Larger Megaspores.
- A sporophyte (diploid) phase produces haploid spores by meiosis.
- These spores germinate to produce haploid Gametophyte
The Gametophytic Plant Body:-
The life cycle is diplohaplontic that shows heteromorphic alternation
of sporophyte and gametophyte which are independent of each other.
- The gametophyte is called Prothallus,they are all small ,they have simple structure and short life cycle,and are all
produced by germination of haploid spore .
- Homosporous species produce bisexual gametophytes whereas
heterosporous species produce unisexual gametophytes. Microspore
germinates to produce male gametophyte and megaspore female
- Gametophytes show the ventral and dorsal differentiation.
- The gametophyteis usually
photosynthetic (not in heterosporous members) and reproduces sexually, that is oogamous.
- The male sex organs are Antheridia that produces sperm by mitosis
- Female sex organs are
Archegonia produces eggs by mitosis They are multicellular with sterile jacket, but without
- Fertilization occurs in presence of water and takes place in the venter
- The diploid zygote develops into embryo in archegonial venter. The
embryo grows by mitosis into a sporophyte (the typical "fern" plant).
Difference between pteridophytes and
Predominant plant body is a sporophyte(2n)
||It is gametophyte (n)|
|Sporophyte is well differentiated into root, stem and leaves
||Sporophyte is only differentiated into foot, seta and capsule
|Gametophyte is always thalloid
||While it is leafy or thalloid|
|Archegonia and antheridia are reduced as campare to bryophytes
||More developed |
|Have well developed vasculature(vascular tissues)
||There is no vasculature|
||Roots absent, rhizoids present |
Difference between Pteridophytes and Gymnosperms
Sporophyte body is not large and tree like
||Sporophyte plant body is large and tree like in most of the Gymnosperms|
|Roots are adventitious.
||They arise from the radical (tap root) in gymnosperms
|Pteridophytes may be homosporous or heterosporous
||While all gymnosperms are heterosporous.|
|Generally secondary growth is absent
|| secondary growth is present.|
|Pollination is absent.
||Pollination is present.|
|Siphonogamy (fertilization with pollen tube) is absent
|| Siphonogamy (fertilization with pollen tube) is present |
Psilotum - in tropical and subtropical regions of Asia and the Americas
- Tmesipteris in tropical regions of the South
Pacific and Australia.
- Both genera occur on rich soil or as epiphytes.
- Group consisting of two extant genera, Psilotum and Tmesipteris, in
- These plants are primitive in structure: Psilotum lacks both roots and leaves and is structurally
to the fossil genus Rhynia.
- Recent molecular systematic studies suggest that the family is actually related to
- Tmesipteris has a more complex morphology in that it has structures on the aerial shoot that are foliar. Both Psilotum and
Tmesipteris have compound sporangia called synangia.
- This group is characterized by its lack of true leaves and roots, the presence of
underground stems or rhizomes, and erect, dichotomously-branched, photosynthetic stems.
- While both genera have aerial
arising from stems embedded in its substrate, they both lack roots.
- The rhizomes are infected with mycorrhizae .
occurs dichotomously by the division of the apical meristem and is most obvious in the areal branches of Psilotum.
Psilotum lacks true leaves, it possesses leaf-like extentions of the stem called enations.
- Because these lack
are not considered leaves.
- However, in Psilotum complanatum, a vascular trace occurs below the enations.
of Tmesipteris are vascularized.
- The gametophytes of both genera are non-photosynthetic and live in association with a fungus (epiphyte).
Sporophytes of Psilotum
- Psilotum is a low-growing plant devoid of any roots or leaves.
primarily of regularly dichotomously branched stems.
- Besides reaching about 1 foot above the soil surface, the stems
extend beneath the soil, branching
a network of smaller stems to hold the plant erect and absorb water and minerals for nutrition.
- Rhizomes:- These
underground stems, called
rhizomes. The rhizome is covered by epidermal cells that grow out into the soil, which are called rhizoids act like root
Because P. nudum's primitive system of absorbing nutrients and water through rhizomes is terribly inefficient,
Psilotum establishes an obligatory mutualistic relationship with a fungus that penetrates the rhizome. The fungus
derives some photosynthetic products from Psilotum and Psilotum is benefited by the increased surface area provided by the
fungal hyphae. This allows more water and nutrients in the soil to enter Psilotum. This relationship is called a mycorrhizal
relationship and since the fungus penetrates the cells of Psilotum it is considered to be endomycorrhizal. The fungus is better
at nutrient and water uptake than the Psilotum, and so its relationship allows both organisms to exist in arid environments.
- Aerial stems :-The rhizome system will send up aerial shoots. These stems grow up into the light and carry out
photosynthesis.The stem has ridges and valleys. The stomata (white spots) occur in the depressions. This provides a small
amount of protection from excess water loss when the Stomata are open.
Stomata:-To control water loss, P. nudum has
stomata (like all modern plants). Each stoma consists of two guard cells, which open and close depending on how much water is
in the plant. If there is plenty of water (for instance after rain), then the stomata will open and allow water within the
plant to be released in a process called transpiration. (The releasing of water allows more water to be 'sucked in' through the
rhizomes). If it is dry, then the stomata are closed and water loss is reduced.
- Leaves:- The leaves are reduced to
irregularly placed scales — probably as a response to unfavourable conditions. The Leaves originate at the Shoot Apical
Meristem. They have a Spiral arrangement (Alternate). The
above ground stems of Psilotum nudum have
small scale-like appendages called enations and clustered of yellow thick-walled, spherical 3-lobed,homosporous
spore-producing structures sporangia called synangia.
- Synangia(singular: synangium):-
The synangia produce microscopic, single-celled reproductive units called spores. When mature, the synangia open, releasing the
spores to be dispersed by the wind.
The dust-like spores can be carried for miles to produce the next generation.
Internal Structure (Anatomy):-The vascular system in both rhizome and aerial stems is a protostele. It is a solid
vascular cylinder consisting of a fluted cylinder of xylem (actinostele) completely surrounded by phloem. The xylem maturation
is exarch. As in the dicot root vascular cylinder (solid, ridged xylem cylinder with radial xylem/phloem arrangement), Psilotum
has a similar format.
- Internal structure of rhizome:-
The Psilotum Rhizome has the most simple kind of tissue organization with Xylem at the center, surrounded by Phloem,
Endodermis, Ground Tissue and Epidermis.
The Xylem of Psilotum contains Tracheids which are similar to those seen in other Vascular Plants. These have characteristic
Secondary Wall thickenings, unlike the Hydroids . The Phloem contains Sieve Elements which are similar to those seen with other
Vascular Plants, unlike Leptoids which do not have all the features of Sieve Elements.
- Anatomy of the aerial Stem :-
- The stem has a central core of vascular tissue (protostele) which is usually
- The erect aerial stems are almost circular (as seen in a transverse section) as they emerge from
the soil but change in shape towards the distal region, are often pentagonal towards the first dichotomy and in the most distal
portions become triangular.
- Epidermis heavily cutinized but interrupted here and there with depressed stomata and
- Cortex massive, differentiated into three zones, outer 1-3 layered, photosynthetic; middle sclerenchymatous, 1-4 layered;
composed of thin many layered parenchymatous cells; endodermis with casparian thickenings follows.
thick-walled cells in the center of this core are sometimes considered to be pith, in which case the vascular arrangement would
actually be a siphonostele.But, the stem has
a central core of vascular tissue (protostele) which is usually lobed. Stele is an actinostele with prominent xylem rays
(appearing like a star); thin walled cells not clearly
separable into phloem and pericycle surround the xylem.
- In the transitional region at the base of the aerial axes the
increases in amount, becomes medullated splitting into a variable number of separate strands.
- Enations :-
The thin scale-like leaves or appendages are spirally arranged, composed
entirely of photosynthetic tissues and covered by an epidermis. They contribute little to the nutrition of the plant since they
are without veins (vascular supply) and have no stomata. Interestingly, the only other
psilophyte, Tmesipteris has stem appendages that intergrade from enations to microphylls (having a single vascular bundle
entering and running the length of the leaf).
Spore production :- The three-lobed sporangium located
on short, lateral branches. Each sporangium
is subtended by bracts.
Development of sporangium:-
- The development of the sporangium is of the
eusporangiate type (i.e. the entire sporangium develops from
a group of sporangial initials).
- The superficial cells divide by periclinal divisions into an inner and outer cell. In
subsequent development, the sporogenous tissue arises from the inner products of the initial periclinal divisions, and most of
the sporangial wall arises from the outer.
- The wall of an eusporangiate sporangium is always more than one layer of
thick.The sporangial wall, usually one to two cells thick.
- Only part of the sporogenous tissue becomes
sporocytes. Sporocytes undergo meiosis to produce a tetrad of spores.
- Part of the sporogenous tissue degenerates to
slimy mass of material around the developing sporocytes. This is called a plasmodial tapetum.
- The tapetum serves as
tissue for the sporocytes and later, spore tetrads. The tetrad of spores breaks apart and individual spores are released
the sporangium ruptures.
Gametophytes of Psilotum :-The spore:- When the synangia mature, they open to
release yellow to whitish
Psilotum is homosporous. The spore tetrads may be tetrahedral or even isobilateral.The mature kidney shaped spores has an outer
thin and reticulate exine and an inner intine. It is uninucleate, about 0.65-0.32 mm in size and with granular and food laden
The spore germinate very slowly.Th early stages of germination studied by Darnell Smith (1917):-
exine ruptures along the median slit.
- The intine protrudes out as a small globular outgrowth which gradually increases
in size.The protruded part is later separated from the basal part, which is with the spore wall by transverse wall.
- The young prothallus is without chlorophyll and at this stage consists of two cells.
- Germinating in soil they must
quickly establish a relationship with soil fungi.
These live symbiotically for the
the life of the gametophyte in the subterranean environment.
The mature prothallus:-
- The haploid
germinate into a haploid prothallus (n), the gametophyte. Psilotum gametophytes are non–photosynthetic and subterranean.It is
cylindrical in shape .
- It is radially symmetrical and dichotomously
branched saprobes that have mycorrhizae for
absorption of nutrients and water
- In form they resemble the rhizome and are very difficult to distinguish from young rhizomes.
They thus support the “Homologous Theory” of alternation of generations according to which sporophyte and gametophyte are
merely different manifestations of a single plant body.
- In size the gametophyte ranges from 0.5 to 2mm.
gametophyte is usually dark brown due to the presence of endophytic
fungus and cuticularization of superficial cell walls.
- Internal structure of gametophyte:-Gametophytes of Psilotum
also contain small amount of vascular tissue
similar to that of sporophytes. A central mass of tracheids is surrounded by phloem and an endodermis.
- Gametophytes are bisexual; antheridia are large and slightly emergent.
- The antheridia and
archegonia are essentially on the fern pattern and are borne intermingled over the
surface of the prothallus.
- The jacket of the antheridium is composed of 10-12 cells in a single layer; at the apex is an opercular cell. The jacket
encloses multicilliate sperms.
- Archegonia have a small neck composed of 4 rows of neck cells with 5 cells in each row,
two neck canal cells, a venter canal cell and an egg. At maturity all cells except the egg break down forming a fluid which
oozes out of the opening of the archegonia.
- This fluid attract the sperms
- Fertilization:- When free water
is available, the sperm are released and swim to the
archegonia and fertilize the egg.The sperm finds the egg by a chemotactic response.
resulting zygote grows into an embryo that differentiates into a complete
Embryogenesis:- In the fertilized egg or the zygote the first wall is transverse to the long axis of the archegonia,
hypobasal and an epibasal cell. The hypobasal cell through further divisions gives rise to a foot. The cells invade the
prothallus tissue with haustorial processes as in Anthoceros. The epibasal cell, divides by a vertical wall, followed by a
transverse wall, resulting in the formation of a quadrant. By repeated divisions a cylindrical structure, the embryonic
rhizome, is formed. No root primordia are present. The rhizome develops rhizoids and becomes infected with a fungus as it
emerges from the gametophyte. Some of the branche tips become negatively geotropic and produce erect aerial axes. Around this
time the embryonic stem usually separates from the foot (which remains within the gametophyte).
Cytology:- P. nudum has two cytological races. A diploid with a chromosome number n=52; and a tetraploid with n=104.
gametophytes of the tetraploid tracheids are present).
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