Plant Evolution and Diversity Notes

History of Plant Emergence

  • Life in the sea was restrictive, leading to the invasion of land due to limited food sources.
  • The colonization of land began with cyanobacteria, followed by green algae and fungi, which grew symbiotically.
  • Most of these organisms were found near the water’s edge.
  • Organic wastes and remnants accumulated, creating soil.
  • Mosses and other plants became established in the newly formed soil, further enriching it.
  • Today, a rich diversity of green plants makes carbon compounds out of water and carbon dioxide using sunlight as an energy source.

Plant General Characteristics

  • All plants are multicellular photoautotrophs.
  • Plants have chlorophylls a & b.
  • They possess accessory pigments like yellow and orange carotenoids.
  • Carbohydrates are stored as starch.
  • Cellulose is the major component of the cell wall.

Plant Ancestors

  • Recent data suggests land plants descended from a group of green algae called charophytes or stoneworts.
  • Charophytes possess a polymer called sporopollenin that prevents exposed zygotes from drying out.

Adaptations on Land - Apical Meristem

  • Root systems (underground):
    • Allows plants to colonize lands.
    • Root systems have many underground absorptive structures, resulting in a large surface area for absorption of soil water and dissolved mineral ions.
    • In many root systems, the root system also anchors the plant.
  • Shoot systems (aboveground):
    • Evolvement of shoot systems where the stems and leaves are adapted for exploiting sunlight and absorbing carbon dioxide from the air.
    • Extensive growth of stems and branches became possible due to the strengthening of cell walls afforded by deposits of lignin.
  • Lateral meristem:
    • Vascular tissues become increasingly extensive.
    • Xylem conducts water and dissolved ions (minerals), and phloem conducts products of photosynthesis such as dissolved sugars.
  • Water conservation:
    • Crucial for life on land, which is further from water sources.
    • Shoots become protected by a cuticle, which is a waxy coat that helps conserve water on hot and dry days.
    • Stomata, which are tiny openings across the surfaces of leaves and some stems, evolved to control carbon dioxide absorption and restrict evaporative water loss.
  • Secondary compounds:
    • Plants produce secondary compounds such as alkaloids, terpenes, tannins, and phenolics like flavonoids that defend against herbivores and parasites, absorb harmful UV radiation, and act as signals in symbiotic relationships with beneficial soil microbes.
  • Shift to diploid dominance:
    • The shift to diploid dominance was an adaptation to land colonization.
    • The diploid phase (sporophyte) is fully adapted to life in terrestrial habitats.
    • The diploid phase (sporophyte) dominates most plant life cycles.
  • Evolution of pollen and seeds:
    • In algae and simple vascular plants, the spores are all alike, a condition called homosporous.
    • In gymnosperm and angiosperm lineages, the spores are differentiated into two types, known as heterosporous. This characteristic is the forerunner for seed evolution.
    • Male gametophytes (microspores/pollen grains) are released from the parent plant to be carried to the female gametophytes via air currents, birds, or insects.
    • Female gametophytes (megaspores) remain in the plant and are surrounded by protective tissues and nutritious tissue; upon fertilization, they will produce seeds.

Plant Groups

  • Nonvascular plants (bryophytes) have a simple internal transport system and lack true roots, stems, or leaves.
  • Seedless vascular plants (ferns) have internal tissue systems (vascular system) that conduct water and solutes through roots, stems, and leaves but do not bear seeds.
  • Seed-bearing vascular plants (Gymnosperms), such as conifers, have a true vascular system and bear seeds.
  • Seed-bearing and flowering vascular plants (Angiosperms) have a true vascular system and bear seeds and flowers.

Ten Phyla of Extant Plants

Refer to Table 29.1 for a detailed list, including:

  • Nonvascular Plants (Bryophytes):
    • Phylum Hepatophyta (Liverworts): 9,000 species
    • Phylum Anthocerophyta (Hornworts): 100 species
    • Phylum Bryophyta (Mosses): 15,000 species
  • Vascular Plants:
    • Seedless Vascular Plants:
      • Phylum Lycophyta (Lycophytes): 1,200 species
      • Phylum Pterophyta (Pterophytes): 12,000 species
    • Gymnosperms:
      • Phylum Ginkgophyta (Ginkgo): 1 species
      • Phylum Cycadophyta (Cycads): 130 species
      • Phylum Gnetophyta (Gnetophytes): 75 species
      • Phylum Coniferophyta (Conifers): 600 species
    • Angiosperms:
      • Phylum Anthophyta (Flowering plants): 250,000 species

Bryophytes

  • Can be divided into three groups:
    • Mosses (Bryophyta)
    • Liverworts (Hepatophyta)
    • Hornworts (Anthocerophyta)
  • Considered as lower plants because:
    • Lack of vascular system, absence of roots and shoots; small morphology permits water and nutrients to be transported without a vascular system. Rhizoids are used for water and nutrient absorption.
    • Cannot survive in non-watery habitats because the sperm is motile by means of flagella.
    • Small and attached sporophyte.
  • Features that help them adapt:
    • The existence of a cuticle, a waxy coat with numerous stomata that prevent excessive water loss.
    • A cellular protective jacket surrounds the sperm-producing and egg-producing parts to prevent drying out.
    • They have large gametophytes that are not dependent on sporophytes; instead, the embryo sporophyte begins life inside a female gametophyte.

Bryophytes - Mosses

Mosses have archegonia (female) and anteridia (male).

Bryophytes - Liverworts

Liverworts have female and male gametophytes and reproduce via sporophytes.

Bryophytes - Hornworts

  • Hornworts have a sporophyte and gametophyte.

Ecological and Economic Importance of Mosses

  • Mosses are capable of inhabiting diverse and sometimes extreme environments but are especially common in moist forests and wetlands.
  • Some mosses might help retain nitrogen in the soil.

Bryophytes Conclusion

  • Bryophytes, the simplest plants:
    • are still dependent on water for fertilization.
    • have a dominant gametophyte generation.

Pteridophytes

  • Most of the seedless plants that flourished in the past are extinct; descendants of them are ferns, lycophytes, horsetails, whisk ferns.
  • Differ from bryophytes in three key aspects:
    • Their sporophytes have well-developed vascular tissue for proper support and conduction.
    • Their sporophytes develop independently from the gametophytes.
    • Their sporophytes have a longer life phase in the life cycle.
  • Pteridophytes are the ‘amphibians’ of the plant kingdom because their sporophytes can live on land, but their gametophytes cannot because they lack vascular tissues, and the male gametes need water to reach the eggs; therefore, none of them has fully escaped the aquatic habitats.

Transport in Xylem and Phloem

  • Vascular plants have two types of vascular tissue: xylem and phloem.
  • Xylem conducts most of the water and minerals and includes dead cells called tracheids.
  • Phloem consists of living cells and distributes sugars, amino acids, and other organic products.
  • Water-conducting cells are strengthened by lignin and provide structural support.
  • Increased height was an evolutionary advantage.

Pteridophytes Classification

  • Can be divided into two phyla:
    • Phylum Pterophyta includes ferns, horsetails, and whisk ferns and their relatives.
    • Phylum Lycophyta includes club mosses, spike mosses, and quillworts.

Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns

  • Ferns are the most diverse seedless vascular plants, with more than 12,000 species.
  • They are most diverse in the tropics but also thrive in temperate forests.
  • Horsetails were diverse during the Carboniferous period but are now restricted to the genus Equisetum.
  • Whisk ferns resemble ancestral vascular plants but are closely related to modern ferns.

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts

  • Giant lycophytes thrived for millions of years in moist swamps.
  • Surviving species are small herbaceous plants.
  • Club mosses and spike mosses have vascular tissues and are not true mosses.

Pteridophytes - Lycophytes (Lycophyta)

  • Sellaginella is a lycophyte group that produces two kinds of spores (male and female) in the same strobilus, a condition called heterosporous. This later will result in seed evolution, the characteristics of higher plants.
  • In some species, the tiny gametophytes are non-photosynthetic, causing them to be nurtured by symbiotic fungi.

Rise of Seed-bearing Plants

  • Bryophytes and pteridophytes are seedless plants; therefore, they reproduce by releasing spores that will grow into gametophytes.
  • The most successful of the vascular plants are the seed-bearing species because they have escaped dependency on water for fertilization, relying instead on air currents and insects.
  • Seed-bearing plants also have a dominant sporophyte phase.
  • Gymnosperms and angiosperms are main groups and differ from the seedless vascular plants in two ways:
    • They are heterosporous - They produce microspores (male) which develop into pollen grains that carry the sperm to the female structures to accomplish pollination, without relying on a water medium. This allows pollination even during prolonged drought. Pollen can be dispersed at a greater distance.
    • They also produce megaspores (female) that develop into ovules and, after fertilization, become seeds. Seeds can resist harsh environments (by being dormant), and offspring dispersal is better.
  • They possess water conservation traits:
    • Compared to seedless vascular plants, these plants have water-conserving traits such as:
      • thicker cuticles and stomata recessed below the surface of the leaf
      • the pollen have sporopollenin to protect against desiccation
      • the dormant seed possesses a food supply and thick seed coat
    • These traits gave gymnosperms and angiosperms the competitive advantage in land colonization.

Gymnosperms – Plants with Naked Seeds

  • Gymnosperm sporophyte stages are conspicuous trees and shrubs.
  • Unlike seeds of flowering plants, which are enclosed inside a chamber called the ovary, the seeds of gymnosperms are unprotected (naked seeds) that are not protected within ovaries.
  • Can be divided into four phyla: Coniferophyta, Cycadophyta, Gingkophyta, and Gnetophyta.

Gymnosperm - Coniferophyta

  • Largest gymnosperm phyla.
  • The conifers are woody trees and shrubs that have needle-like or scale-like leaves – thicker cuticles and smaller surface area.
  • Most are evergreen (shed some leaves all year long), while a few are deciduous (shed their leaves in the fall).
  • The pine trees produce two kinds of cones/strobilus:
    • Small male cones produce sporangia, which yield microspores that develop into pollen grains (male gametophyte).
    • Larger female cones produce ovules that yield megaspores (female gametophytes).

Cycads (Cycadophyta)

  • Are palm-like trees.
  • They bear massive cone-shaped strobili that produce either pollen (transferred by air currents or insects) or ovules.
  • Individuals have large cones and palm-like leaves.
  • These thrived during the Mesozoic, but relatively few species exist today.

Gingkos (Gingkophyta)

  • They were a diverse group in dinosaur times, but the only surviving species today is Gingko biloba.
  • They are remarkably hardy, deciduous, and show resistance to insects, disease, and air pollutants.
  • It has a high tolerance to air pollution and is a popular ornamental tree

Gnetophytes (Gnetophyta)

  • The most unusual gymnosperms, where they live in tropical and desert areas.
  • Two examples are the shrubby Ephedra and Welwitschia mirabilis that could be found in the desert area.
  • This phylum comprises three genera: Gnetum, Ephedra, and Welwitschia.

Angiosperm – Phylum Anthophyta

  • Are seed-bearing and flowering plants.
  • Flowers are specialized reproductive structures (gametophyte).
  • Of all the divisions of plants, angiosperms are the most successful and the most diverse.
  • Most flowering plants co-evolved with pollinators such as insects, bats, and birds. These pollinators withdraw nectar or pollen from a flower and transfer the pollen to the female reproductive parts.

Angiosperm Groups

  • There are four major groups of flowering plants:
    • Basal Angiosperm, such as the Water Lily
    • Magnoliids, including magnolias, avocados, nutmeg, and black pepper plants.
    • Eudicots, including familiar shrubs, trees (except conifers), and herbaceous (non-woody) plants.
    • Monocots, including grasses, lilies, and other major food crop grains.

Angiosperm - Basal Angiosperm

Basal Angiosperm is survived by the oldest lineage, a species called Amborella trichopoda, which is a small shrub.

Angiosperm - Magnoliids

Magnoliids include woody and herbaceous species.

Angiosperm - Monocots and Eudicots

  • Monocots:
    • one cotyledon
    • floral parts in threes
    • parallel leaf veins
    • pollen grain has one pore or furrow
    • vascular bundles throughout stem's ground tissue
  • Eudicots:
    • two cotyledons
    • floral parts in fours or fives
    • netlike leaf veins
    • pollen grain has three pores or furrows
    • stem's vascular bundles arranged in a ring

Angiosperm Life Cycle

  • In the angiosperm life cycle, the diploid sporophyte has extensive root and shoot systems where it also retains and nourishes the gametophytes (in the flowers) and disperses its sperm-bearing pollens (from the flowers).
  • In an angiosperm life cycle, double fertilization is one of the distinctive characteristics.

Angiosperm - Flower (Gametophyte) Structure

  • Stamen has two parts:
    • Anther
    • Filament
  • Pistil has three parts:
    • Stigma
    • Style
    • Ovary

Angiosperm - Pollen Development

  • Pollen sacs within the anther contain numerous diploid cells called the microsporocytes; each will undergo meiosis to produce four haploid cells called the microspores.
  • Each microspore will develop into a pollen grain that consists of the tube cell and the generative cell. The generative cell will divide to form two nonmotile sperm cells.

Angiosperm - Ovule Development

  • Each young ovule within an ovary contains a diploid cell called the megasporocyte that will undergo meiosis to produce four haploid megaspores.
  • Three of these will disintegrate, and the fourth will undergo mitosis without any cytoplasmic division to produce a multicellular female gametophyte called an embryo sac.
  • The embryo sac is embedded in the ovule and typically contains seven cells with eight haploid nuclei.
  • Six of them (including the egg cell) have single nuclei each, while a large central cell has two nuclei called the polar nuclei.

Angiosperm - Fertilization

  • The egg and both of the polar nuclei participate directly in fertilization.
  • Right after pollination, which is the transfer of the pollen grain to the female part, fertilization, which is the fusion of the male and female gamete, happens.
  • In double fertilization, the egg fuses with one sperm cell, forming a zygote (fertilized egg) that eventually develops into a multicellular embryo in the seed.
  • The two polar nuclei fuse with the second sperm cell, forming a triploid nutritive tissue called the endosperm. This tissue surrounds the developing embryonic plant in the seed and supplies them with nourishment.
  • After double fertilization, the ovule develops into a seed, while the surrounding ovary develops into a fruit.

Summary of Evolution of Gametophyte and Sporophyte Phases

  • In most plants, there is an alternation between the development of sporophytes and gametophytes.
  • Life cycles change as plants radiated into higher and drier places.

Angiosperm Alternation of Generation

  • In simple plants, gametophytes (haploid) dominate the life cycle. The sperm released from the gametophyte is motile and needs a watery environment for fertilization. Examples are bryophytes and pteridophytes.
  • However, in pteridophytes, the gametophyte and sporophyte develop independently of each other – the gametophyte in water and the sporophyte on land. The sporophyte is more dominant as compared to the gametophyte.
  • In complex land plant cycles (seed-bearing), the diploid phase (sporophyte) dominates. Both the sporophyte and gametophyte develop on land, and they have fully escaped water dependency.