Spermatophyta, Gymnosperms & Angiosperms

Spermatophyta

• Monophyletic group
• Synapomorphy: Seed formation
  • A seed consists of an embryo (immature diploid sporophyte) surrounded by nutritive tissue and enveloped by a seed coat.

Other Major Characteristics of Seed Plants

• Reduced gametophytes
• Heterospory
• Ovules
• Pollen

Advantages of Reduced Gametophytes

• Gametophytes develop within the walls of spores.
• Spores are retained within tissues of the parent sporophyte.
• Advantages:
  • Protection from environmental stresses
  • Shielded from UV radiation
  • Gametophyte can directly obtain nutrients from sporophyte
  • Better chance of survival

Dominance of Gametophyte vs. Sporophyte

• Gymnosperm
  • Microscopic female gametophytes (n) inside ovulate cone
  • Microscopic male gametophytes (n) inside pollen cone
• Angiosperm
  • Microscopic female gametophytes (n) inside parts of flowers
  • Microscopic male gametophytes (n) inside parts of flowers

Heterospory in Seed Plants

• Ancestors of seed plants were likely homosporous, while seed plants are heterosporous.
• Megasporangia:
  • Produce megaspores that give rise to female gametophytes.
• Microsporangia:
  • Produce microspores that give rise to male gametophytes.

Ovules and Production of Eggs

• An ovule consists of a megasporangium, megaspore, and one or more protective integuments.
• Gymnosperm megaspores have one integument.
• Angiosperm megaspores usually have two integuments.
• A seed develops from the whole ovule.

Pollen and Production of Sperm

• Microspores develop into pollen grains, which contain the male gametophytes.
• Pollination: transfer of pollen to ovules
• Advantage of pollen: transfer of sperm to ovules without water
• Pollen tube: Allows sperm to reach the egg within the ovule

Advantages of Seeds Over Spores

• Seeds may remain dormant for days to years until conditions are favorable for germination.
• Seeds have a supply of stored food.
• Seeds may be transported long distances by wind or animals.

Gymnosperms vs. Angiosperms

• Gymnosperms:
  • Bear “naked” seeds typically on cones.
  • Seeds are exposed on sporophylls that form cones.
• Angiosperms:
  • Seeds are found in fruits, which are mature ovaries.

Plant Groups

• Nonvascular plants (bryophytes)
• Seedless vascular plants
• Gymnosperms
• Angiosperms

Gymnosperm Phyla

1. Cycadophyta (cycads)
2. Ginkgophyta
3. Gnetophyta
4. Coniferophyta: Conifers, such as pine, fir, and redwood

Phylum Cycadophyta

• Modern seed plants with flagellated sperms
  • Sperm emerge from pollen grains, then swim in the fluid produced by the ovule.
• 130 species mainly in dry tropics and sub-tropics
• “sago palm”
  • Starch is harvested from the trunk and used as edible food.
  • Commonly used in landscaping and houseplants.

Phylum Ginkgophyta

• Consists of a single living species, Ginkgo biloba (maiden hair tree)
• Deciduous native of China
• High tolerance to air pollution, insects, and diseases makes it a popular ornamental tree.
• Dietary supplements made from Ginkgo may slow memory loss in people with Alzheimer’s disease.

Phylum Gnetophyta

• Comprises three genera:
  • Gnetum
  • Ephedra
  • Welwitschia
• Species vary in appearance; some are tropical, while others live in deserts.
• Ephedra is sold as an herbal stimulant and weight loss aid.

Phylum Coniferophyta

• Largest of the gymnosperm phyla with 600 species of trees and shrubs with woody cones.
• Most are evergreens and can carry out photosynthesis year-round.
• Needle- or scale-like leaves with a thick cuticle are resistant to drought and cold weather.

Life Cycle of a Pine

• Three key features:
  1. Dominance of the sporophyte generation
  2. Development of seeds from fertilized ovules
  3. Transfer of sperm to ovules by pollen
• The pine tree is the sporophyte and produces sporangia in male and female cones.
• Small cones produce microspores called pollen grains, each containing a male gametophyte.
• The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes.
• It takes nearly three years from cone production to mature seed.

Angiosperms (Flowering Plants)

• Seed plants with reproductive structures called flowers and fruits.
• Key adaptations:
  • Flowers
  • Fruits
• Classified in a single phylum, Anthophyta (from the Greek anthos for flower).
• Most widespread and diverse of all plants.
  • Angiosperms began to replace gymnosperms near the end of the Mesozoic era.
  • Angiosperms now dominate most terrestrial ecosystems.

Flower

• Specialized shoot with up to four types of modified leaves:
  • Sepals: Enclose the flower
  • Petals: Brightly colored and attract pollinators
  • Stamens: Produce pollen
  • Carpels: Produce the ovules
• Stamen:
  • Filament
  • Anther (microsporangium): Pollen production
• Carpel:
  • Stigma
  • Style
  • Ovary: Egg/ovum production

Fruits

• Typically consists of a mature ovary but can also include other flower parts.
• Fruits protect seeds and aid in their dispersal.
• Mature fruits can be either fleshy or dry.

Angiosperm Life Cycle

• Microsporangium in anther produces microsporocytes (2n).
• Microsporocytes undergo meiosis to produce microspores (n).
• Microspores develop into pollen grains (n) containing generative and tube cells.
• Megasporangium in ovule produces megasporocytes (2n).
• Megasporocytes undergo meiosis to produce megaspores (n).
• Surviving megaspore develops into a female gametophyte (embryo sac).
• Pollination occurs; pollen tube grows toward the ovule.
• Two sperm nuclei (n) are discharged into the embryo sac.
• One sperm fertilizes the egg (n) to form a zygote (2n).
• The other sperm fertilizes the central cell (2n) to form endosperm (3n).
• The ovule develops into a seed containing the embryo (2n), endosperm (3n), and seed coat (2n).

Angiosperm Diversity

• Angiosperms comprise more than 250,000 living species.
• Previously, angiosperms were divided into two main groups:
  • Monocots (one cotyledon)
  • Dicots (two cotyledons)
• Cotyledon (“seed leaf”): embryonic leaf, the first to arise from the germinating seed.

Monocots vs. Dicots: DNA Evidence

• DNA studies suggest that monocots form a monophyletic clade, but dicots are polyphyletic.
• The clade eudicot (“true” dicots) includes most dicots.
• The rest of the former dicots form several small lineages.
  • Basal angiosperms: include the flowering plants belonging to the oldest lineages.
  • Magnoliids: share some traits with basal angiosperms but evolved later.

Plant Evolutionary Tree

• The flowering plants (angiosperms) diverged from their common ancestors with the gymnosperms over 200 million years ago.
• The “basal angiosperms” Amborellaceae, Nymphaeaceae, and Austrobasileyales diverged first, followed by the magnoliid clade before the eudicots and the monocots diverged about 140 million years ago.
• The dicots are a polyphyletic group.

Basal Angiosperms

• Originated when the angiosperms were still a young clade.
• Their ancestors became reproductively isolated from the other early flowering plants before distinctive angiosperm traits had originated.
• These groups continued to evolve as the other clades evolved.
• Amborellaceae, Nymphaeaceae, and Austrobaileyales are the extant descendants of that original divergence.

Magnoliids (or Magnoliidae)

• A group of about 9,000 species of flowering plants.
• Characterized by:
  • Trimerous flowers
  • Pollen with one pore
  • Branching-veined leaves

Characteristics of Monocots vs Eudicots