Gymnosperms

Gymnosperms are seed plants whose ovules are not enclosed by an ovary wall, so their seeds remain naked. These notes on gymnosperms cover their habit, roots, stems and leaves, and their heterospory with male and female cones. They then explain the pollen grain, the ovule and female gametophyte, and the steps from pollination to seed formation. They also compare gymnosperms with bryophytes and pteridophytes. Pairs to match, memory tricks and NEET-format questions make the topic quick to revise.

1. Gymnosperms: Meaning and Habit

• ★ Exam imp The word gymnosperm comes from gymnos (naked) and sperma (seeds).
• In gymnosperms the ovules are not enclosed by any ovary wall.
• The ovules remain exposed, both before and after fertilisation.
• The seeds that develop after fertilisation are not covered: they are naked.
• Gymnosperms include medium-sized trees, tall trees and shrubs.
• The giant redwood tree Sequoia is one of the tallest tree species.

2. Roots, Stems and Leaves

• The roots are generally tap roots.
• ★ Exam imp In some genera the roots have a fungal association called mycorrhiza, for example Pinus.
• ★ Exam imp In others, such as Cycas, small specialised roots called coralloid roots occur.
• Coralloid roots are associated with ${\mathrm{N}}_2$-fixing cyanobacteria.
• The stems are unbranched (Cycas) or branched (Pinus, Cedrus).
• The leaves may be simple or compound.
• In Cycas the pinnate leaves persist for a few years.
• Gymnosperm leaves are well adapted to withstand extremes of temperature, humidity and wind.
• In conifers, the needle-like leaves reduce the surface area.
• Their thick cuticle and sunken stomata also help to reduce water loss.

3. Spores, Cones and the Pollen Grain

• ★ Exam imp Gymnosperms are heterosporous. They produce haploid microspores and megaspores.
• The two kinds of spores are produced within sporangia borne on sporophylls.
• The sporophylls are arranged spirally along an axis, forming lax or compact strobili or cones.
• Strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
• The microspores develop into a male gametophytic generation.
• This male gametophyte is highly reduced and confined to a limited number of cells.
• ★ Exam imp This reduced male gametophyte is called a pollen grain.
• Pollen grains develop within the microsporangia.
• Cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
• ★ Exam imp The male and female cones may be borne on the same tree, as in Pinus.
• In Cycas, male cones and megasporophylls are borne on different trees.

4. The Ovule and Female Gametophyte

• The megaspore mother cell is differentiated from one of the cells of the nucellus.
• The nucellus is protected by envelopes.
• ★ Exam imp The composite structure, the nucellus with its envelopes, is called an ovule.
• The ovules are borne on megasporophylls, which may be clustered to form the female cones.

1. A cell of the nucellus differentiates into the megaspore mother cell.
2. The megaspore mother cell divides meiotically to form four megaspores.
3. One of the megaspores, enclosed within the megasporangium, develops further.
4. It forms a multicellular female gametophyte that bears two or more archegonia (female sex organs).
5. The multicellular female gametophyte is retained within the megasporangium.

5. Pollination, Fertilisation and Seed

• ★ Exam imp Unlike bryophytes and pteridophytes, the gametophytes of gymnosperms are not free-living.
• They remain within the sporangia retained on the sporophytes.

1. The pollen grain is released from the microsporangium.
2. Pollen grains are carried by air currents.
3. They come in contact with the opening of the ovules borne on megasporophylls.
4. A pollen tube carrying the male gametes grows towards the archegonia in the ovule.
5. The pollen tube discharges its contents near the mouth of the archegonia.
6. After fertilisation, the zygote develops into an embryo.
7. The ovules develop into seeds. These seeds are not covered.

6. Gymnosperms Compared with Bryophytes and Pteridophytes

• ★ Exam imp Three groups of plants bear archegonia: bryophytes, pteridophytes and gymnosperms.

7. Exam Essentials: Traps, Exceptions, Numbers and Pairs

Numbers to Remember

• 4 megaspores from one megaspore mother cell (meiosis).
• 1 megaspore develops into the female gametophyte.
• 2 or more archegonia on each female gametophyte.
• Cycas pinnate leaves persist for a few years.
• 2 kinds of spores: microspores and megaspores.
• 3 plant groups bear archegonia: bryophytes, pteridophytes, gymnosperms.

Examples to Remember

Pairs to Match

8. Quick Revision

• Gymnosperms: ovules not enclosed by an ovary wall; seeds naked.
• Medium or tall trees and shrubs; Sequoia (giant redwood) among the tallest.
• Tap roots; mycorrhiza in Pinus; coralloid roots with nitrogen-fixing cyanobacteria in Cycas.
• Stem unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves simple or compound; Cycas pinnate leaves persist a few years.
• Conifers: needle leaves, thick cuticle, sunken stomata reduce water loss.
• Heterosporous: haploid microspores and megaspores on spirally arranged sporophylls in cones.
• Male (microsporangiate) strobili make pollen grains, the highly reduced male gametophytes.
• Female (macrosporangiate) strobili carry ovules on megasporophylls.
• Pinus: both cones on one tree; Cycas: male cones and megasporophylls on different trees.
• Ovule = nucellus + envelopes; megaspore mother cell gives four megaspores by meiosis.
• One megaspore forms a multicellular female gametophyte with two or more archegonia.
• Gametophytes are not free-living; pollen travels by air; pollen tube carries male gametes.
• Zygote becomes the embryo and ovule becomes the naked seed.

9. Solved Examples

10. Practice Questions

Common Mistakes to Avoid