3. PLANT KINGDOM
PTERIDOPHYTES
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They include horsetails and ferns.
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They are found in cool, damp, shady places.
Some flourish well in sandy-soil conditions.
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Evolutionarily, they are the first
terrestrial plants to possess vascular tissues (xylem & phloem).
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In bryophytes, the dominant phase in the
life cycle is the gametophyte. In pteridophytes, the dominant phase (main plant
body) is a sporophyte. It is differentiated to true root, stem & leaves. These organs have well-differentiated vascular
tissues.
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The leaves in pteridophyta are small (microphylls)
as in Selaginella or large (macrophylls) as in ferns.
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Economic importance: They
are used for medicinal purposes and as soil-binders and ornamentals.
REPRODUCTION:
- The sporophytes bear sporangia that
are subtended by leaf-like appendages called sporophylls. In some cases,
sporophylls may form distinct compact structures called strobili or cones
(E.g. Selaginella, Equisetum).
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Sporangia produce spores by meiosis in
spore mother cells.
- The spores germinate to give
inconspicuous, small, multicellular, free-living, mostly photosynthetic
thalloid gametophytes called prothallus.
- Prothallus requires cool, damp, shady
places to grow. Also, it needs water for fertilization.
So, the spread of pteridophytes is limited and restricted to narrow
geographical regions.
- The gametophytes (prothallus) bear male and
female sex organs called antheridia and archegonia, respectively.
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Water is needed for transfer of antherozoids
(male gametes from antheridia) to the mouth of archegonium.
- Antherozoid fuses with the egg in the archegonium
to form zygote. Zygote develops to a multicellular well-differentiated sporophyte.
- Most of the pteridophytes produce similar
kinds of spores (homosporous plants). Others produce two kinds of
spores, macro (mega) & micro spores. They are heterosporous.
E.g. Selaginella & Salvinia.
- The megaspores & microspores
germinate and give rise to female and male gametophytes, respectively. The
female gametophytes are retained on the parent sporophytes for variable
periods.
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Within female gametophytes, zygotes develop
into young embryos. This event is a precursor to the seed habit. It is
considered as an important step in evolution.
- The
pteridophytes have 4 classes:
1.
Psilopsida: E.g. Psilotum
2.
Lycopsida: E.g. Selaginella, Lycopodium
3.
Sphenopsida: E.g. Equisetum
4. Pteropsida: E.g. Dryopteris, Pteris, Adiantum
