Sunday, July 26, 2020

Plant Kingdom - Notes | Class 11 | Part 3: Pteridophytes

3. PLANT KINGDOM

PTERIDOPHYTES

-    They include horsetails and ferns.

-    They are found in cool, damp, shady places. Some flourish well in sandy-soil conditions.

-    Evolutionarily, they are the first terrestrial plants to possess vascular tissues (xylem & phloem).

-    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.

-    The leaves in pteridophyta are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.

-    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).

-    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.

-    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.

-    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  


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