Plant Adaptations and Life Cycles

Challenges of Terrestrial Life & Plant Adaptations

• Scarcity of Water:

  • Impact: Affects general survival and reproduction (e.g., flagellated sperm need water to swim).

  • Adaptations:

    • Cuticle: A waxy, water-resistant substance secreted by outer cell layers, covering all land plants (ancestral character) to prevent desiccation. It is not cellular itself but produced by cells.

    • Stomata: Openings or pores in the cuticle, flanked by two guard cells, allowing for gas exchange (like CO_2 for photosynthesis). Most plants have stomata that can open and close. Liverworts, however, only have fixed pores without guard cells, making them highly susceptible to water loss.

    • Size/Circulatory System: Plants either remain very small, relying on diffusion and direct contact with water for all cells, or develop a complex circulatory (vascular) system for larger bodies (a later adaptation in vascular plants).

• Lack of Structural Support/Buoyancy:

  • Impact: Air provides less buoyancy than water, making it challenging to grow upright.

  • Adaptations:

    • Small Size/Thin Bodies: Nonvascular plants remain small and thin, often sprawling, relying on simple cell walls for some rigidity, instead of strong internal support systems.

    • Specialized Cells: Even without a true vascular system, some nonvascular plants have specialized, larger cells with thicker walls that provide a degree of structural support and facilitate limited water movement via diffusion.

• Intense UV Radiation:

  • Impact: Underwater, UV light is filtered; on land, it's intense and damaging to DNA.

  • Adaptations:

    • Flavonoids: UV-absorbing compounds present in plant cells that protect DNA from damage, similar to melanin in human skin. An absorption spectrum shows high UV absorption by flavonoids at low wavelengths.

Opportunities of Terrestrial Life

• Unfiltered Sunlight: Provides abundant photosynthetically active wavelengths.

• Increased Carbon Dioxide (CO2): Easier access to CO2 from the air compared to dissolved CO_2 in water, which is a raw material for photosynthesis.

• Rich Soil Resources: As plants colonize and decompose, they contribute to the formation of nutrient-rich soil, offering higher concentrations of nutrients compared to diffuse nutrients in water.

• Abundant Space and Resources: Initially, land environments presented fewer competing organisms (mostly fungi, bacteria, archaea), offering vast unoccupied space and resources for colonizing organisms.

Reproductive Adaptations & Alternation of Generations

• Alternation of Generations: The life cycle of land plants, involving two multicellular stages: a haploid gametophyte and a diploid sporophyte.

• Sporangia: Tough, protective containers in which haploid spores are produced. This protects spores until release, which is crucial in a dry terrestrial environment.

• Sporopollenin: A water-resistant molecule coating spores, allowing them to remain dormant and survive dry conditions after release until favorable conditions for germination occur.

• Fertilization in a Dry Environment: Green algae and some early land plants have flagellated (swimming) sperm, requiring water for fertilization. Adaptations arose to facilitate this or evolve beyond it.

• Gametangia: Protective containers for gametes:

  • Archegonia: Female gametangia, each containing a single egg. The egg remains stationary within the archegonium.

  • Antheridia: Male gametangia, producing many flagellated sperm.

• Multicellular Embryo: Land plants produce multicellular embryos that are initially dependent on and attached to the parent gametophyte, receiving resources (analogous to a maternal-fetal connection via an umbilical cord). This feature gives plants the name embryophytes. Extensive surface area contact between maternal gametophyte and embryo cells facilitates nutrient transfer.

• Numerical Advantage of Multicellular Diploid Stage: The evolution of a multicellular sporophyte stage (a diploid stage that undergoes mitosis before meiosis) offers a numerical advantage. Instead of a diploid zygote immediately undergoing meiosis to produce 4 spores, a zygote undergoing mitosis first to form a multicellular sporophyte can then produce a much larger number of spores through meiosis (e.g., 1 zygote to 4 spores vs. 1 zygote + 2 mitotic divisions to a 4-cell embryo producing 16 spores). This increases reproductive output and successful dispersal of offspring.

Life Cycle of Green Algae (Sister Group to Land Plants)

• Green algae are a diverse group; those most closely related to land plants typically have only one multicellular stage: the haploid gametophyte.

• The multicellular gametophyte produces eggs (stationary) and flagellated sperm (swimming).

• Fertilization occurs in water, forming a diploid, single-celled zygote.

• The zygote immediately undergoes meiosis to produce haploid, flagellated (swimming) spores.

• These spores swim away, germinate, and grow by mitosis into new multicellular gametophytes.

• Crucially, green algae lack a multicellular diploid (sporophyte) stage.

Moss Life Cycle (Example of Nonvascular Plants)

• Gametophyte Dominant: The gametophyte is the larger, longer-lived, and free-living stage, while the sporophyte is smaller, temporary, and dependent on the gametophyte.

• Gametes: Female gametophytes produce eggs in archegonia; male gametophytes produce sperm in antheridia. Some gametophytes can be bisexual.

• Fertilization: Flagellated sperm require a film of water to swim from the antheridia to the eggs in the archegonia. This makes reproduction dependent on moist conditions.

• Zygote & Sporophyte Development: After fertilization, the single-celled diploid zygote forms within the archegonium on the female gametophyte. The zygote then undergoes mitosis to develop into a multicellular diploid sporophyte, which remains attached to and dependent on the gametophyte.

• Sporophyte Structure: In mosses, the sporophyte typically consists of a stalk with a capsule (sporangium) at its apex.

• Spore Production & Release: Inside the sporangium, spore mother cells undergo meiosis to produce haploid spores. These spores are coated in sporopollenin and are dispersed, often by air.

• Protonema: When a spore lands in a suitable moist environment, it germinates (grows by mitosis) into a filamentous, two-dimensional structure called a protonema.

• Gametophyte Maturation: The protonema continues to grow by mitosis, eventually developing into a mature, three-dimensional gametophyte.

Nonvascular Plants (Bryophytes)

• Paraphyletic Group: Includes liverworts (Hepatophyta), mosses (Bryophyta), and hornworts (Anthocerophyta). They share a common ancestor but do not include all its descendants (i.e., vascular plants).

• Evolutionary Context: These lineages diverged early (around 470 ext{ million years ago}) and retain many ancestral characteristics of land plants, though they have continued to evolve.

• Gametophyte Dominant Life Cycle: As described for mosses, the gametophyte is the prominent, free-living stage, and the sporophyte is dependent and ephemeral.

• Lack of Vascular Tissue: This is their defining characteristic and why they are called