Biol120 - Lecture 14: Seed Plants Gymnosperms

Origins of Seed Plants

• Appeared $\approx 360\, \text{mya}$; derived from progymnosperms (e.g., Elkinsia)
• Represent extreme heterospory: distinct micro‐ and megaspores
• Trend: reduction & retention of gametophytes within sporophyte tissues

Advantages of Seeds

• Protective coat + stored food $\Rightarrow$ survival in dormancy & harsh conditions
• Dispersal by wind, water, animals $\Rightarrow$ reduced parent–offspring competition
• Independence from surface water for fertilisation

Shared Derived Features of Seed Plants

• Ovule = megasporangium $+$ integument $\rightarrow$ seed after fertilisation
• Pollen (male gametophyte) with sporophyte-derived wall; enables aerial dispersal; non-motile or reduced sperm
• Microscopic gametophytes retained & protected within sporophyte
• Secondary growth (wood) common; allows tall, long-lived forms

Gymnosperms – Major Lineages

• Cycadophyta (cycads)
• Ginkgophyta (Ginkgo biloba)
• Gnetophyta (Ephedra, Welwitschia, Gnetum)
• Coniferophyta (conifers: Pinaceae, Cupressaceae, Araucariaceae, Podocarpaceae)

Key Gymnosperm Features

• “Naked seeds” borne on cone scales (megasporophylls)
• Sporangia on modified leaves (sporophylls) arranged in cones
• Dominant sporophyte with reduced gametophytes; female gametophyte provides nutritive tissue
• Some (cycads, ginkgo) retain flagellated sperm, but transport occurs within pollen tube after pollination

Seed Development Essentials

1. Megasporocyte ($2n$) in ovule $\xrightarrow{meiosis}$ 4 megaspores; 1 survives
2. Surviving megaspore $\xrightarrow{mitosis}$ female gametophyte (produces archegonia)
3. Microsporocyte ($2n$) in pollen cone $\xrightarrow{meiosis}$ microspores $\rightarrow$ pollen grain (male gametophyte)
4. Pollination $\rightarrow$ pollen tube delivers 1 sperm to egg (fertilisation $\approx 15$ months in pines)
5. Fertilised ovule $\rightarrow$ seed: embryo ($2n$) + food supply ($n$) + seed coat ($2n$)

Pine (Pinus) Life-Cycle Highlights

• Separate male & female cones on same tree (monoecious)
• Time frame: cone initiation to seed release up to $2$ years
• Seed dispersal $\rightarrow$ germination $\rightarrow$ new sporophyte

Economic & Cultural Importance

• Conifers supply $\approx 60\%$ of industrial wood (timber, pulp, resins)
• Iconic species: Wollemia nobilis, Agathis australis (kauri – boat building, Māori waka)

Key Terms

• Ovule, Integument, Pollen, Seed
• Heterospory, Microspore, Megaspore
• Microsporangium, Megasporangium
• Endosperm (in angiosperms; gymnosperm nutritive tissue = female gametophyte)