Describe the problems generated by life on land for the green plant lineage. How have these problems been overcome?



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Describe the problems generated by life on land for the green plant lineage. How have these problems been overcome?”
Intro:

  • Land plants all monophyletic called the Embryophytes ~480 million years ago

  • Only one transition (whereas many in animals) – suggests many problems to be overcome whilst remaining relatively immobile.

  • During transition phase, exposed to:

    • Extreme temperature fluctuations

    • High UV exposure

    • Highly desiccating environment

    • Altered nutrient uptake and gas exchange

    • Predation by animals

  • Plants adapted to environmental constrains by large changes in morphology and physiology.



Aquatic  Aerial environment:

  • Extreme desiccation = primitive trait evolved in bryophytes

  • Liverworts:

    • Hugs moist ground/saturated air near moist soil.

    • Large SA:V

    • Absorb water by osmosis

    • Amphious – no waterproofing.

    • Can suspend metabolism in dry conditions and recovery quickly following rehydration – cell structure and photosynthetic machinery protected (mosses too)

      • Rehydrins

  • Hornworts:

    • Waterproof cuticle composed of cutin (hydrophobic polymer)

    • Stomata allow gas exchange – aperture controlled by guard cells which regulate water loss

    • Inside, cells for pathway of cells like alveoli for efficient diffusion.



Unidirectional transport system:

Mosses: (improved in vascular plants)

    • Xylem is a conducting tube of cells

    • Cellulose cell walls strengthened by polypropanoid polymer (lignin)

  • Cohesion and adhesion of water molecules with each other and wall

  • Negative pressure draws water up through transpiration

  • Phloem in vascular plants carries photosynthate to root hair cells



Reproduction:

  • Undergo sporic meiosis

    • Diploid sporophyte  (meiosis) spores  (mitosis) gametophyte  (mitosis)  gametes  (fusion)  diploid sporophyte

  • Aquatic plants release sperm and eggs for fertilisation in water

  • Can reproduce clonally to propagate genes or sexually to increase genetic diversity




    • Gametophytes in all embryophytes

      • Sterile pockets called archegonia contain and protect gametes

      • Eggs in archegonia

      • Sperm in antheridia

      • Released at separate times

    • Sperm must swim in film of water to reach egg – gametes fuse to form zygote

    • Sterile cells of archegonium nourish and protect young sporophytes

    • Transport cells with high SA:V transport nutrients from gametophyte to sporophyte

    • = DIPLOID DEPENDENCE (came earlier in ferns)




  • Psilotum = vascular fern-like plants

    • Sporophyte produces spores coated in decay-resistant sporopollenin

    • Germinate underground – small gametophyte

    • Archegonium releases gametes

    • Sporophyte grows into light (autotrophic therefore stops being dependent on the gametophyte)




  • In bryophytes, sporophytes remain parasitic to gametophyte – lack functional chloroplasts

  • In ferns, gymnosperms and angiosperms, sporophytes evolved to be independent of gametophyte – autotrophic!

  • Selection against aerial sperm-producers intense due to desiccation – resulted in transition to diploid-dominant lifecycle as opposed to haploid-dominant.



Evolution of seeds:

  • Spermatohytes began to dominate Earth ~250 mya - best shown in angiosperms

  • During seed reproduction, fertilisation is truly internal

  • After fertilisation, tissues surrounding the embryo harden to produce a seed from the whole ovule. Therefore completely isolated from the external environment and so protected against desiccation.

  • Seeds can enter dormancy for 10s or even 100s of years before geminating when conditions are more optimal.

  • Allowed plants to inhabit previous inhospitable environments (e.g. arid deserts)

  • Seeds increase success rate of fertilised gametophytes

  • Endosperm provides a nutrient store within embryo

  • Competitive advantage over existing terrestrial plants – increased biodiversity.



Control of fertilisation:


  • Controlled by sporophyte through maternal tissue of the style

  • Prevents self-fertilisation and ensures outcrossing

    • Increases genetic diversity

  • This is a form of sexual selection

Conclusion:


  • Evolution of land plants resulted in increasing levels of complexity

    • From bryophytes to complex gymnosperms and angiosperms present today

  • Throughout history they’ve evolved a diverse array of specific morphological, physiological and behavioural adaptations to allow them to tolerate hostile terrestrial conditions.

  • Their colonisation of land and subsequent diversification caused a series of environmental changes, resulting in the generation of many different habitats and development of entire ecosystems.


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