Introduction to botany  © punam kumar

 

CHAPTER 17:

Pteridophytes structure and reproduction

P>Sections --"A"

17.1.The stele or vascular system of Pteridophytes

17.2. Difference between homospory & heterospory

17.3. What is Pteridophytes ?- Introduction

17.4. Evolution and Classification of Pteridophytes

Sections --"B"

17.5.. Habitats of Pteridophytes

17.6. The Sporophytic Plant Body of Pteridophytes

17.7. The Gametophytic Plant Body of Pteridophytes

17.8. Difference between pteridophytes and bryophytes

17.9. Difference between Pteridophytes and Gymnosperms

Section "C" Subsection---"1"

17.10. Habitat of Whisk Ferns(Psilotopsida)

17.11. General characteristics of Whisk Ferns(Psilotopsida)

17.12. Sporophytes of Psilotum

Section "C" Subsection---"2"

17.13.Internal Structure (Anatomy) of Psilotum

17.14.Development of sporangium of Psilotum

Section "C" Subsection---"3"

17.15.Gametophytes of Psilotum

17.16. Reproduction of Psilotum

17.17. Embryogenesis of Psilotum

Section "D" Subsection---"1"

17.18. Equisetum belongs Class Equisetopsida

17.19. Vegetative structure(Sporophytes) of Equisetum

17.20. Stem anatomy of Equisetum

Section "D" Subsection---"2"

17.21. Reproduction of Equisetum

17.22. Sexual reproduction of Equisetum

Section "E" Subsection---"1"

17.23.Members of class pteridopsida

17.25. General characteristics of ferns

17.26. Internal structure (anatomy) of ferns

Section "E" Subsection---"2"

17.27. Reproduction of ferns

17.28. Gametophyte of ferns

17.29. Life cycle of ferns

Section "A"

17.1.The stele or vascular system of Pteridophytes

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17.2. Difference between homospory & heterospory


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17.3. 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.

17.4. 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 appeared. 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 Pteridophyta 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.


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Section "B"

17.5. 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 :-

17.6. The Sporophytic Plant Body:-

  1. 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).
  2. 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.
  3. The sporophyte performs vegetative reproduction and asexual reproduction.
  4. Vegetative reproduction takes place by vegetative buds that develop on the rhizome or by fragmentation of rhizome.
  5. Asexual reproduction takes place by means of spores produced inside the sporangia.
  6. The sporangia are borne on lower surface or in axils of fertile leaves called sporophylls.
  7. The sporangia are borne singly or in groups called Sori.
  8. 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.
  9. A sporophyte (diploid) phase produces haploid spores by meiosis.
  10. These spores germinate to produce haploid Gametophyte

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17.7. The Gametophytic Plant Body:-

  1. 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 .
  2. Homosporous species produce bisexual gametophytes whereas heterosporous species produce unisexual gametophytes. Microspore germinates to produce male gametophyte and megaspore female gametophyte.
  3. Gametophytes show the ventral and dorsal differentiation.
  4. The gametophyteis usually photosynthetic (not in heterosporous members) and reproduces sexually, that is oogamous.
  5. The male sex organs are Antheridia that produces sperm by mitosis
  6. Female sex organs are Archegonia produces eggs by mitosis They are multicellular with sterile jacket, but without stalks.
  7. Fertilization occurs in presence of water and takes place in the venter of archegonium.
  8. The diploid zygote develops into embryo in archegonial venter. The embryo grows by mitosis into a sporophyte (the typical "fern" plant).
The life cycle is diplohaplontic that shows heteromorphic alternation of sporophyte and gametophyte which are independent of each other.

17.8. Difference between pteridophytes and bryophytes

Pteridophytes Bryophytes
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 present Roots absent, rhizoids present

17.9. Difference between Pteridophytes and Gymnosperms

Pteridophytes 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

Section "C" Subsection---"1"

Psilotopsida.Whisk Ferns:-

17.10. Habitat:-

17.11. General characteristics:-

Psilotum


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17.12. Sporophytes of Psilotum

Section "C" Subsection---"2"

17.13. Internal Structure (Anatomy) of Psilotum:-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.

Spore production :- The three-lobed sporangium located on short, lateral branches. Each sporangium is subtended by bracts.

17.14. Development of sporangium:-

The tetrad of spores breaks apart and individual spores are released when the sporangium ruptures.

Section "C" Subsection---"3"

17.15. Gametophytes of Psilotum :-The spore:- When the synangia mature, they open to release yellow to whitish spores. 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 cytoplasm.

The spore germinate very slowly.The early stages of germination studied by Darnell Smith (1917):-

The mature prothallus:-

17.16. Reproduction:-


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17.17. Embryogenesis:- In the fertilized egg or the zygote the first wall is transverse to the long axis of the archegonia, forming a 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. (In certain gametophytes of the tetraploid tracheids are present).

Section "D" Subsection----"1"

17.18. Equisetum:- Class Equisetopsida contains a single order Equisetales.Found in the Upper Devonian, best in Carboniferous from whence its decline started, until in Triassic it was represented by a few genera. The order contains a single surviving family Equisetaceae and a sole living genus Equisetum.

Habitats :-Equisetum with 15 species of almost world-wide distribution and confined to N. Temperate regions, though some are met with in Tropics also, except Australia. From India four species are known: E. arvense, E. diffusum, E. palustre and E. ramosissimum.Species of Equisetum are usually known as “horse tails” or “pipes” or “scouring rushes”.

Habit:-Plants small to large, terrestrial, usually grow in wet or marshy places or in open, sunny sand banks along rivers and margins of lakes.

17.19. Vegetative structure(Sporophytes) of Equisetum - MorphologyThe plant body of Equisetum is differentiated into stem, leaves and roots.

17.20. Stem anatomy: -Trassverse section of stem at internode is wavy in outline with prominents ridges and grooves(furrows).It shows different zone:-

Section "D" Subsection---"2"

17.21. Reproduction:-Equisetum is homosporous. Sporangia are clustered in cones (strobili) at the tips of vegetative shoots in some Equisetum species, while in some species have a simple non-green, short-lived, unbranched shoot with terminal strobili.

Structure of strobilus:-The strobilus has a massive central axis, which consist of tightly packed appendages called sporangiophores.The central axis of the cone is divided into internodes and nodes, on nodes are arranged in vertices or whorls of sporangiophores that alternate in successive nodes. No bracts or leaves are present in between.


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The sporangiophore consists of a central stalk and a broad, peltate, hexagonal disc-like structure (cap) which bears on its under surface 5-10 pouch-like pendant sporangia. Each sporangiophore is supplied by a vein which branches distally. When young the discs are tightly packed up but at maturity the internodes elongate slightly and separate the sporangiophores. The stalk also grows at a right angle so that the sporangia are approximately perpendicular to the surface of the soil.

Development of Sporangia:- Eusporangiate type. A single superficial cell (adjacent 5-10 cells may also add to the sporogenous tissue) on the peltate disc acts as sporangial initials. The first division is periclinal. Further periclinal and anticlinal divisions in the outer cell form a multi-layered jacket (wall) the inner cells of which break down to form the tapetum. The inner cells by further divisions in all directions give rise to the sporogenous tissue which develops into the spore-mother-cells. Some of these abort and add to the tapetum for the nourishment of the developing spores. The spore-mother-cells undergo meiosis (reduction division) to produce four spores. The sporangia dehisce longitudinally.

Spores:-Mature spores bear four long, spoon-shaped appendages known as elaters; these are originally coiled around the spore itself but uncoil when the spores are released.Coiling or uncoiling of elaters depends upon the humidity. Elaters help in the dispersal of the spore, and also help in finding a suitable substratum for germinating due to their hygroscopic nature. Each spore has a three layered wall, intine, exine and a perispore (or perine).The spores contain chlorophyll(green colour).The spores germinate within a few days.

Gametophyte:-

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17.22. Sexual reproduction:-The gametophytes are unisexual or bisexual but protogynous(producing female gametes before male ones ).

Embryogenesis:- The fertilization of the egg results in the formation of the zygote and marks the end of the gametophytic generation which began with the formation of the spores.


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Section "E" Subsection---"1"

17.23. PteridopsidaMembers of class pteridopsida evolved during the Devonian period(405 years ago) and diversified further during Carboniferous period(345 million year ago) ensuring that it was the dominant vegetation on earth.They became extinct during Permian period.The surviving true ferns again became prominent from the Mesozoic periods(Triassic -245-208 million years ago and the Jurassic 208-146 million years ago). These are the true ferns.In the number of genera and species, members of the Filicopsida (or the ferns) are next to the flowering plants amongst the terrestrial plants. It includes about 31-64 (or even more) families, 300-443 genera and 9,000-15000 species.

HabitatsThese are most abundant and diverse in the tropics, but they are also well represented in the temperate zone.

Habit-The members grow as perennial herbs,shrubs and as a tree.Many members grow as epiphytes on the leaves of tropical flowering plants, or tiny aquatic Azolla to large tree ferns with leaves reaching 15 feet in length and whose roots and stems are among the toughest “wood” known.

17.24. Classification of Ferns Two characteristics are used to classify the fern orders:

. The Osmundaceae which possesses intermediate features between these two groups.

17.25. General characteristics of ferns:- Sporophyte:-The sporophyte is the asexual or diploid(2n) stage of the pteridophyte life-cycle. It is mostly large and conspicuous, always green and photosynthetic, long-live and produces the spores in special fruiting bodies; the spores are shed to produce the next gametophyte generation in the cycle.

Section "E" Subsection---"2"

17.27. Reproduction:-

17.28. Gametophyte:-The fern gametophyte, often called a prothallium, or prothallus, is small, always short lived and inconspicuous but independent from the sporophyte and photosynthetic. It is often heart shaped and have an apical Cell in the notch.Gametophytes are without vascular tissue and often only one cell thick for the most part, tapering to many cells thick at the centre. They are attached to the substrate by fine multicellular root hairs or rhizoids, and often resemble thallose liverworts. Sexual reproduction:-The prothallus has a single set of chromosomes ( haploid) and is the sexual part of the life cycle.Most ferns are homosporous.On the undersurface of the prothallus, develops the sex organs, The antheridia are near the base Antheridia and archegonia may be on the same gametophyte or on separate gametophytes.

17.29. Life cycle:-

 

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