Describe the problems generated by life on land for the green plant lineage. How have these problems been overcome?
|“Describe the problems generated by life on land for the green plant lineage. How have these problems been overcome?”
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
Hugs moist ground/saturated air near moist soil.
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)
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
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
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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