Seedless Vascular Plants (Pteridophytes)

Home > Biology 103: Botany > Exam Three

Learning Objectives

1. What evolutionary adaptations allowed vascular plants to be so successful?
2. How does sexual reproduction occur in vascular plants?
3. Understand alteration of generations in vascular plants in terms of sporophyte and gametophyte stages.

Solutions

1. Vascular tissue colved the problem of moving water and nutrients through plants thanks to lignin. The gametophyte generation is also smaller, less vulnerable, and more protected that previous generations.
2. Seedless vascular plants are "oogamous", meaning they have immotile eggs and sperm which either swim or are carried in water. Earlier seedless vascular plants are homosporous, where gametophytes can form both antheridia or archgonia. Later plants are heterosporous, where only one or the other can be formed.
3. Difficult to write in sentences, see below:
• Multicellular embryo
• Free-living sporophytes prominent
• Sporophyte meiosis → 1n spores
• 1n spores mitosis → gametophytes
• Gametophytes produce 1n gametes
• Gametes + fertilization = 1n zygote
• Zygote mitosis → sporophyte
• Sporophyte meiosis → 1n spores

Notes

Plant Evolutionary Tree

• Has:
• chlorophyll a & b
• land-dwelling adaptations (cuticle, gametophyte, and embryo)
• vascular systems (xylem & phloem)
• Does not have:
• Seeds
• Flowers
• Pteridophytes are ferns, horsetails, and lycophytes
• Lycophytes are clubmoss, spikemoss, and quillwarts
• With all vascular plants, the porophyte stage of alterations of generation is the most prominent/dominant stage
• Vascular systems solved the problem of moving water and nutrients through plants that had evoled lignin (cell wall support tissue)

Evolution of Vascular Plants

• Plant trends: towards independence from water for substinence and reproduction
• Alteration of generations from gametophyte (1n) to sporophyte (2n)
• multicellular embyro
• free-living sporophytes more prominent in vascular plants

Evolutionary Adaptations of Vascular Plants

• Development of xylem and phloem (vascular tissue)
• Ability to synthesize lignin (cell walls, support)
• Sporophyte with multiple sporangia
• Roots instead of rhyzoids
• Gametophyte generation diminished in size to increase protection and decrease vulnerability
• All contribute to water use efficiency

Seedless Vascular Plant Body

• Previously:
• Dichotomous branching
• Lacked roots and leaves
• Structures relied on turgor pressure
• Development of:
  • Roots, stems/shoots, and leaves
  • 3 distinct and functionally unique tissues:
  • Dermal (epidermis)
  • Vascular (xylem and phloem)
  • Ground tissue
  • Primary growth in apical meristems
  • Limited secondary growth
  • Lignified cell walls
  • Thicker stems and roots
  • Xylem: tracheids
  • Conductive tissue with lignified walls
  • Move water and support plant
  • Complex leaf structure
• Early pteridophytes had microphylls
• Single strand of vascular tissue
• Later developed megaphylls
• Leaf blade with branching veins (vascular system)

Ferns - Vascular Bundles

• Xylem and phloem form vascular bundles and connect the leaves, underground stem, and root
• Only primary xylem = no woody tissue
• Large size, but not as big as plants with secondary growth

Reproductive System

• Seedless vascular plants are oogamous
• Large, nonmotile eggs
• Small sperm that swim or are carried in water
• Seedless vascular plants are homosporous
• Each spore can produce bisexual gametophytes that can form either archegonia or antheridia
• Later, seedless vascular plants are heterosporous
• Each spore can produce a specific type of sporangia
• Microspores: sperm
• Megaspores: eggs
• Trends in plant evolution
• Smaller and smaller gametophytes
• More and more dependency on sporophyte for nutrition and protection
• Not free-living like bryophytes and pteridophytes

Phyla

• Pteridophytes are generally accepted as:
• Ferns and horsetails
• Lycophytes

Ferns

• Typically does not have aerial stem, the leaves are directly from an underground stem called a rhizome. This is the sporophyte (2n) phase, so the sporangium forms on the bottom of the leaves
• Sporangium: spre (n) produced by meiosis
• Gametophytes (n) are heart-shaped and tiny; normally found in moist soils since they need water for reproduction
• Very diverse group, ranging from a few millimeters to more than 6 feet tall
• Often found in forest understory, but that doesn't mean they only grows in shady environments. They can often be found in swamps, sunny and open fields, and attached to another plant (called an epiphyte)

Equisetum (Horsetail Ferns)

• Another "living fossil"
• Reduced whorled leaves and usually non-photosynthetic
• Single, nonbranching stem

Psilotum (Whisk Ferns)

• Considered "primitive" ferns because they do not have roots or leaves, so photosynthesis happens on the stem
• Dichotomous branching
• Always happens on a terminal bud
• Not derived from axillary buds

Lycophytes: Lycopodium and Selaginella

• No notes on slide
• Images look like one, singular, non-woody rosemary