Introduction to botany  © punam kumar

 

CHAPTER 17:

Pteridophytes structure and reproduction

Section "A"

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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 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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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 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.
  3. All vegetative parts possess vascular tissues organized into steles or vascular bundles. So, pteridophytes are are first tracheophytes in evolution of plant kingdom.
  4. The sporophyte performs vegetative reproduction and asexual reproduction.
  5. Vegetative reproduction takes place by vegetative buds that develop on the rhizome or by fragmentation of rhizome.
  6. Asexual reproduction takes place by means of spores produced inside the sporangia.
  7. The sporangia are borne on lower surface or in axils of fertile leaves called sporophylls.
  8. The sporangia are borne singly or in groups called Sori.
  9. 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.
  10. A sporophyte (diploid) phase produces haploid spores by meiosis.
  11. These spores germinate to produce haploid Gametophyte

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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.

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

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

Psilotopsida.Whisk Ferns:-

Habitat:-

General characteristics:-

Psilotum


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Sporophytes of Psilotum

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.

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

Development of sporangium:-