Plant Evolution and Growth

Overview of Plants Without Seeds

  • Land plants can be vascular or nonvascular.

  • Red Algae and Green Algae are also relevant.

Features of Nonvascular Plants

Characteristics of Land Plants

Alternations of Generations

Features of Vascular Plants

Three Clades of Nonvascular and Vascular Plants

Evolution of Seed Plants (Chapter 27) & Plant Body (Chapter 32)

Objectives

  • Understand features that develop in vascular seed plants.

  • Describe major groups of gymnosperms and angiosperms along with their features.

  • Distinguish angiosperms from previous groups.

  • Understand how flowers and fruit evolve.

  • Know the two major clades of angiosperms.

  • Explain how plants grow.

Vascular Seed Plants – Higher Plants

  • Surviving vascular seed plants fall into two groups:

    • Gymnosperms (pines and cycads)

    • Angiosperms (flowering plants)

Feature of Seed Plants

  • Key features include heterospory, seeds, flowers, sporophyte dominance, and reduced gametophyte generations.

  • The sporophyte generation is dominant over the gametophyte generation.

  • Gametophyte generations are reduced further than in ferns.

  • Examples of Gymnosperms are Cycads, Ginkgos, Gnetophytes, and Conifers.

Relationship Between Sporophyte and Gametophyte

  • In mosses, the gametophyte nourishes the sporophyte.

  • In ferns, sporophytes and gametophytes are nutritionally independent.

  • In seed plants, the sporophyte nourishes the developing gametophytes.

Heterospory

  • Earliest groups of gymnosperms had swimming sperm.

  • Later gymnosperms and angiosperms evolved ways of bringing eggs and sperms together, providing a major advantage for existence on land.

Spores of Seed Plants

  • Spores are produced in sporangia by meiosis.

    • Megaspores develop into female gametophytes (megagametophytes) within megasporangia.

    • Microspores develop into male gametophytes (microgametophytes) within microsporangia.

  • Megagametophytes and microgametophytes depend on the sporophyte for food and water.

Female Gametophyte

  • Megaspores develop into megagametophytes within megasporangia.

  • The megagametophyte produces an egg.

  • The integument, layers of sporophyte tissue, surrounds the megasporangium.

  • Megasporangium + integument = ovule à seed.

Male Gametophyte

  • Microspores develop into male gametophytes within microsporangia.

  • Microspores develop into pollen grains.

  • Microgametophyte is within a tough, protective coat of the pollen grain.

  • Pollination occurs when a pollen grain arrives near a female gametophyte.

Male Gametophyte (cont’d)

  • When a pollen grain reaches a female gametophyte, it produces a slender pollen tube that elongates and digests its way through the sporophyte tissue to the megagametophyte.

  • At the tip of the pollen tube, sperm are released, leading to fertilization.

  • The end product is an embryo within a seed.

Seed

  • A complex, well-protected resting stage.

  • May remain viable for many years, germinating only when conditions are favorable.

  • The seed coat protects the embryo from drying and potential predators, a main reason for the evolutionary success of seed plants.

Wood

  • Most ancient seed plants produced wood.

  • Wood = proliferated xylem, which provides support and allows plants to grow above their competitors for sunlight.

  • Not all seed plants are woody.

Major Groups of Gymnosperms

  • Gymnosperms: "Naked Seeds"

  • Seed plants that do not form flowers.

  • Ovules and seeds are not protected by an ovary or fruit tissue.

  • Four major groups of living gymnosperms:

    • Cycads (Cycadophyta)

    • Ginkgos (Ginkgophyta)

    • Gnetophytes (Gnetophyta)

    • Conifers (Coniferophyta)

Features of Gymnosperms

  • Conifers have reproductive structures in male and female cones.

  • Female (seed-bearing) cone = megastrobilus.

  • Seeds are protected by a tight cluster of woody scales (modification of branches).

  • Smaller male (pollen-bearing) cone = microstrobilus.

  • Pollen-bearing microsporangia are covered by scales (modified leaves).

Gymnosperms - Cycads

  • Cycads - Phylum Cycadophyta

  • About 140 species

  • Primitive gymnosperms that bear cones

  • Cycads are palm-like plants with seed-bearing leaves.

Gymnosperms – Ginkgo

  • Ginkgo – Phylum Ginkgophyta

  • Only one living species

  • Male and female trees are determined by X and Y sex chromosomes.

Gymnosperms – Gnetophytes

  • Gnetophytes – Phylum Gnetophyta

  • About 90 species

  • Thrive in arid environments and can be found in desert-like climates, growing low to the ground.

Gymnosperms – Conifers

  • Conifers – Phylum Coniferophyta

  • Most abundant gymnosperms, about 600 species

  • Cone-bearing plants such as pines and redwoods, including the largest living tree, the giant sequoia.

Angiosperms “Flowering Plants”

  • Oldest angiosperm fossils are from the Cretaceous period, 140 million years old.

  • Angiosperms became dominant in only 60 million years.

  • Over 250,000 species exist today.

  • 12 – 30 phyla are recognized today.

Synapomorphies In The Angiosperms

  • Synapomorphies: unique characteristics that define the group.

    • Double fertilization

    • Endosperm (nutritive tissue)

    • Ovules and seeds enclosed in a carpel

    • Flowers

    • Fruits

    • Phloem with companion cells

    • Reduced gametophytes

Features of Angiosperms

  • Double Fertilization:

    • Microgametophyte contains two male gametes.

    • One male gamete + egg à a diploid (2n) zygote.

    • 2nd male sperm + two other haploid (n) cells of female gametophyte à a triploid (3n) nucleus à a triploid tissue called endosperm.

  • Endosperm nourishes the embryonic sporophyte during its early development.

Angiosperms “Enclosed Seeds”

  • Ovules and seeds are enclosed in a modified leaf called a carpel: provides protection and may interact with pollen to prevent self-pollination.

  • Produce flowers and fruits.

Features of Angiosperms

  • Male sex structure = stamens

  • Each stamen à filament + anther

  • Filament à support

  • Anther à pollen-producing microsporangia

Features of Angiosperms

  • Female sex structure = carpels

  • One or more carpels = a pistil.

  • Each pistil à stigma, style, and ovary

  • Stigma à has sticky sugar end that helps pollen stick

  • Style à stalk for support

  • Ovary à contains ovules

Features of Angiosperms

  • Several specialized sterile modified leaves (non-spore bearing)

  • Inner colorful ones = petals (corolla)

  • Outer ones = sepals (calyx)

  • Petals (corolla) à attract pollinators.

  • Sepals (calyx) à protect the flower bud before it opens.

Pollination

  • The carpel receives pollen.

  • The anthers of the stamen bear pollen-producing microsporangia.

  • When the tip of the pollen tube reaches the female gametophyte, sperm are released from the tube, and fertilization ensues.

Angiosperms “Enclosed Seeds”

  • Have xylem (contain vessel elements, fibers, and tracheids) and phloem (sieve tube and companion).

  • Xylem:

    • Vessel elements—specialized for water transportation

    • Fibers—important in supporting plant body, found alongside of tracheids.

  • Phloem:

    • Companion cells—another unique cell associated with sieve tube cells.

Features of Angiosperms

  • Megasporangia + microsporangia = perfect flowers

  • Either megasporangia or microsporangia = imperfect flowers

  • female and male on the same plant = monoecious

  • female and male on different plants = dioecious

Features of Angiosperms

  • Angiosperms produce fruits.

  • Ovary à fruit & Ovule à seed after fertilization

  • Fruit – protects the seeds

  • Fruit – promotes seed dispersal by becoming attached to or be eaten by animals.

Features of Angiosperms

  • Simple fruit – develops from one carpel (one ovary). e.g., plum, cherry

  • Aggregate fruit – develops from several carpels (ovaries). e.g., raspberry

  • Multiple fruit – develops from several flowers with each ovary coming together into a larger fruit. e.g., pineapple

  • Accessory fruit – develops from parts other than carpels, not from the ovary. e.g., strawberry, apple, pear

Flower Form & Evolution

  • Carpels (female reproductive organs) and stamens (male reproductive organs) may have evolved from modified leaves.

Flower Form & Evolution

  • Long styles in pistils and long filaments in stamens are the result of natural selection.

  • Length à↑ pollination à more accessible to insects or to catch the wind.

Coevolution of Angiosperms with Animals

  • Most angiosperms à pollination by animals, insects such as birds, bees, bats, etc.

  • Many flowers entice pollinators with nectar and pollen grains.

  • Plants and their pollinators have coevolved.

  • Some relationships are highly specific (e.g., moth & yucca plant).

Life Cycle of an Angiosperm

  • Double fertilization results in a (2n) zygote and (3n) endosperm.

Origin & Diversity of Angiosperms

  • Most angiosperms are in two clades: monocots and dicots.

    • Monocots – one cotyledon. e.g., grasses, lilies, orchids, palms.

    • Eudicots – two cotyledons. e.g., roses, sunflowers, maples.

  • Cotyledon (seed leaf) – a type of seedling with two embryonic leaves.

Monocots vs. Dicots/Eudicots

  • Monocots

    • One cotyledon

    • Leaves with parallel veins

    • Flower parts usually in threes or multiples of three

    • Vascular bundles in stem scattered throughout the stem

  • Dicots/Eudicots

    • Two cotyledons

    • Leaves usually have netlike veins

    • Flower parts usually in fours or fives or multiples of fours and fives

    • Vascular bundles in stem arranged in a ring

Plant Body

  • All plants à primary plant body and secondary plant body

  • Primary plant growth

    • Results in primary plant body—consists of all the non-woody parts of the plant

    • Elongation of shoots and roots

    • Monocots consist entirely of primary plant body

  • Secondary plant growth

    • Results in secondary plant body—consists of two types of tissues = vascular cambium and cork cambium

      • Vascular cambium à wood and bark

      • Cork cambium à part of bark

    • Increases in diameter/width

    • Roots and stems of dicots develop a secondary plant body

Plant Growth

  • Vascular cambium divides by mitosis to produce woody tissue.

  • Since water availability and seasonal environmental conditions vary throughout the year, different forms of cells are added.

  • Differential growth is seen as rings.

How Do Plants Support Our World?

  • Plants = primary producers.

  • Seed plants are our primary food source.

  • Many medicines come from seed plants.