8.5 Spermatophyta (Seed-Bearing Plants)

Spermatophytes are the most advanced and dominant group of vascular plants, characterized by the production of seeds. They include two major groups: Gymnosperms and Angiosperms.

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Evolutionary Prerequisites for the Seed Habit

Before seeds could evolve, several key adaptations were necessary:

1. Heterospory: Production of two distinct spore types — small microspores (male) and large megaspores (female). This leads to differentiation of male and female gametophytes.
2. Retention of the megaspore: The megaspore is permanently retained within the megasporangium (nucellus) rather than being shed — this is the ovule.
3. Integuments: Protective layers surrounding the megasporangium that develop into the seed coat (testa) after fertilization.
4. Pollen tube (Siphonogamy): Delivers non-motile male gametes to the egg without requiring external water, enabling life in dry habitats.

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Division I: Gymnosperms

General Characteristics (SLO B-11-B-14)

• Naked seeds: Ovules and seeds are borne directly on cone scales, not enclosed within an ovary/fruit.
• Mostly evergreen trees and shrubs with needle-like or scale-like leaves.
• Vascular tissue (xylem and phloem) present; xylem lacks vessels (tracheids only in most).
• Dominant sporophyte generation; gametophyte is microscopic and dependent.
• Fertilization via pollen tube (siphonogamy) — no water required.
• Produce both male cones (microsporangiate) and female cones (megasporangiate).

Classification of Gymnosperms

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Life Cycle of Pinus (SLO B-11-B-15)

Pinus (pine tree) is the classic example of a gymnosperm life cycle. The sporophyte (the tree) is dominant.

Male Cones (Microsporangiate Cones)

• Small, clustered at branch tips.
• Each scale bears two microsporangia.
• Microsporangia undergo meiosis → microspores → develop into pollen grains (immature male gametophyte).
• Mature pollen grain contains: 2 prothallial cells, 1 generative cell, 1 tube cell.
• Pollen is wind-dispersed (anemophily).

Female Cones (Megasporangiate Cones)

• Large, woody, persistent.
• Each scale bears two ovules.
• Each ovule contains: integuments (2), micropyle (opening), nucellus (megasporangium).
• One cell in nucellus undergoes meiosis → 4 megaspores → 3 degenerate, 1 functional megaspore.
• Functional megaspore develops into female gametophyte (endosperm tissue + 2–3 archegonia each containing an egg).

Pollination and Fertilization

1. Pollen grains enter through the micropyle of the ovule (wind pollination).
2. Pollen tube grows slowly through the nucellus (takes ~12–14 months in Pinus).
3. Generative cell divides → 2 sperm cells (non-motile).
4. One sperm fuses with the egg → zygote (2n) → embryo.
5. Integuments harden → seed coat (testa).
6. Seed is released when cone scales open.

$\text{Microspore}\stackrel{\text{mitosis}}{\to }\text{Pollen grain}\stackrel{\text{pollen tube}}{\to }\text{Sperm}+\text{Egg}\to \text{Zygote}\to \text{Embryo}$

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Division II: Angiosperms (Flowering Plants)

General Characteristics (SLO B-11-B-16)

• Seeds are enclosed within a fruit (ripened ovary wall — pericarp).
• Possess flowers as reproductive organs.
• Double fertilization (unique to Angiosperms).
• Dominant sporophyte; highly reduced gametophyte (male = 3-celled pollen grain; female = 7-celled embryo sac).
• Xylem contains vessels (efficient water transport).
• Most diverse group of plants (~300,000 species).

Classification of Angiosperms

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Life Cycle of Angiosperms (SLO B-11-B-17)

Male Gametophyte (Pollen Grain)

• Anther contains microsporangia (pollen sacs).
• Microsporocytes undergo meiosis → microspores → each develops into a pollen grain.
• Mature pollen grain = 3 cells: 1 tube cell + 2 sperm cells (generative cell divides).

Female Gametophyte (Embryo Sac)

• Ovule contains megasporangium (nucellus).
• One megasporocyte undergoes meiosis → 4 megaspores → 3 degenerate, 1 functional.
• Functional megaspore undergoes 3 mitotic divisions → 7-celled, 8-nucleate embryo sac:
  • 3 antipodal cells
  • 2 synergids
  • 1 egg cell
  • 1 central cell with 2 polar nuclei

Pollination

• Transfer of pollen from anther to stigma of the same or different flower.
• Agents: wind (anemophily), insects (entomophily), water, birds.

Double Fertilization (Unique to Angiosperms)

1. Pollen grain germinates on stigma → pollen tube grows through style to ovule.
2. Two sperm cells released:
   • Sperm 1 + Egg cell → Zygote (2n) → develops into Embryo
   • Sperm 2 + 2 Polar nuclei → Primary Endosperm Nucleus (3n) → develops into Endosperm (food reserve)

${\text{Sperm}}_1+\text{Egg}\to \text{Zygote (2n)}\to \text{Embryo}$ ${\text{Sperm}}_2+\text{Polar nuclei (2)}\to \text{PEN (3n)}\to \text{Endosperm}$

Seed and Fruit Formation

• Zygote → embryo (with cotyledons, plumule, radicle)
• Integuments → seed coat (testa)
• Ovary wall → fruit (pericarp)
• Ovule → seed

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Comparison: Gymnosperms vs. Angiosperms