Chapter 3: Plant kingdom
Plant Kingdom | Gymnosperms | Short Note For NEET BIOLOGY| standard 11
Note 10
Gymnosperms
> Gymnos - naked
> Sperma - seeds
> Gymnosperms are plants in which ovules remain open - not covered (enclosed) by any ovary wall
> Ovules remain exposed - both before and after fertilization
> So, seeds developed post-fertilization - are naked (not covered)
> Plant body is sporophyte
> Here, plant body (sporophyte) is well differentiated - well developed roots , stem and leaves are present
>> Size:
> Shrubs
> Medium to tall sized trees
> Tallest tree species- Sequoia (giant redwood tree) is included under Gymnosperms
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| Sequoia tree |
>> Roots:
> Pinus and some other genera have fungal association of roots - Mycorrhiza - this helps in better water absorption
> Cycas and some other genera have small specialized roots - Coralloid roots - roots are associated with Nitrogen (N2) fixing Cyanobacteria - Anabaena and Nostoc
>> Stem:
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| Cycas |
> Branched - Pinus and Cedrus
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| Pinus |
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| Cedrus |
>> Leaves:
> Can be simple or compound
> Cycus - pinnate leaves persist for few years (don't shade faster)
> Leaves of gymnosperms - well adapted for extreme of temperature, humidity and wind
> In conifers (plants of gymnosperms that bear corn) - xerophytic (adaptation for survival in less water) leaves are seen
--> needle like leaves - reduce surface area - reduce water loss through transpiration
--> thick cuticle on leaves - help reduce water loss
--> sunken stomata - water loss reduction
>> Life cycle of Gymnosperms
> Male and female cones are seen in gymnosperms
> These cones can be on same plant eg. Pinus (monoecious)
> Both cones can be on different plants eg. Cycas (dioecious)
> Male cone (Male strobili or Microsporangiate) : made up of many Microphylls set up together spirally
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| Male cone of Pinus |
> Microphylls: these are leaf like appendages - microsporangia is present in their grooves
> Microsporangia: carries group of undifferentiated cells (diploid-2n)
> One of these cells undergoes differentiation - form Microspore Mother Cell (2n)
> Microspore Mother Cell undergoes Meiosis - produces 4 microspores (haploid-n)
> All of the cells multiply and develop to form male gametophytic generation
> Male Gametophyte is highly reduced and is confined to only a limited number of cells.
> This reduced gametophyte is called a pollen grain.
> At maturity, microsporangia will disperse the pollen grains into environment
> Female cone: ( female strobili / megasporangiate) - made up of many megasporophylls arranged spirally
Female cone of Pinus
> Megasporophylls: leaf like appendages- carry ovule (megasporangium) in its groove
> Megasporangium (ovule) : is made up of 2 parts
--> Nucellus - undifferentiated cells which have plenty amount of nutrients
--> integuments - envelopes that cover the nucellus
> One of the nucellus cells differentiate - produce Megaspore Mother Cell
> Megaspore Mother Cell undergoes meiosis - produce 4 megaspores
> Out of 4 megaspores - 3 degenerate and only 1 megaspore is functional
> This one megaspore divide and give rise to female Gametophyte (also called as endosperm)
> Female Gametophyte contains 2 or more Archegonia
> Archegonia is female sex organ - it carries female gamete
> Pollen grain is released in the environment and they are carried through air currents
> They come in contact with ovules on megasporophylls
> Pollen tube is developed - this helps in transferring all its content to the Archegonia in ovule
> Thus, male gamete is transferred to the mouth of Archegonia
> Fertilization occurs and zygote (2n) is produced
> Zygote converts into embryo
> The ovule converts into seed
> Seeds here are not covered
Thank you for reading!
Happy Learning!
Manish Mevada
Urvi Bhanushali
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