Gymnosperms, Angiosperms and Plant Life Cycles: Quick Revision

A brief note on Gymnosperms, Angiosperms and Plant Life Cycles. Read it before a test. Try each question at the end before you open its answer.

Gymnosperms

• Ovules lie naked on megasporophylls, so the seeds have no fruit. Examples: Cycas, Pinus, Cedrus, Ginkgo, Sequoia.
• Tap roots: mycorrhiza in Pinus, coralloid roots with nitrogen-fixing cyanobacteria in Cycas.
• Needle leaves with a thick cuticle and sunken stomata reduce water loss.
• Heterosporous: male cones make pollen grains, the reduced male gametophytes; ovules hold the many-celled female gametophyte with archegonia.
• Pinus bears both cones on one tree; Cycas bears male cones and megasporophylls on different trees.
• The gametophytes never live on their own. The food store of the seed is the haploid female gametophyte.

Angiosperms

• Ovules inside an ovary; seeds inside fruits. Dicotyledons have two cotyledons, monocotyledons one.
• Stamen: filament and anther, which makes pollen. Pistil: stigma, style and ovary.
• Embryo sac: 7 cells, 8 nuclei. The egg and 2 synergids, 3 antipodals, and the central cell with 2 polar nuclei.
• Syngamy: male gamete and egg give the zygote, $2n$.
• Triple fusion: male gamete and two polar nuclei give the primary endosperm nucleus, $3n$.
• Ovule becomes the seed; ovary becomes the fruit; synergids and antipodals degenerate.

Life cycles

• Gametophyte ($n$) makes gametes; sporophyte ($2n$) makes spores by meiosis.
• Haplontic: gametophyte is the main plant; only the zygote is diploid. Spirogyra, Volvox.
• Diplontic: sporophyte is the main plant. Seed plants, Fucus.
• Haplo-diplontic: both many-celled. Bryophytes (gametophyte main), pteridophytes (sporophyte main), Ectocarpus.

A worked example

A plant has $2n = 16$. Give the chromosome numbers in its pollen grain, egg, zygote and endosperm.

1. One set. $n = \dfrac{16}{2} = 8$.
2. Pollen grain and egg. Both are haploid: $8$ each.
3. Zygote. Egg and male gamete: $8 + 8 = 16$.
4. Endosperm. Two polar nuclei and a male gamete: $3 \times 8 = 24$.

Practice with new numbers

A program to try

The program compares the chromosomes in the embryo and the food tissue of a seed, in a flowering plant and in a gymnosperm with the same $n$.

Mistakes to avoid

• Calling a pine cone a fruit. Only an ovary becomes a fruit.
• Giving a gymnosperm seed a triploid endosperm. Its food store is the haploid female gametophyte.
• Counting 8 cells in the embryo sac. There are 7 cells and 8 nuclei.
• Saying that ferns have a main gametophyte. That is mosses; in ferns the sporophyte is the main plant.

Check yourself

1. Name the two partners that help gymnosperm roots, and the plant each one belongs with.

2. Which nuclei of the embryo sac fuse with the two male gametes?

3. In which pattern of life cycle is the gametophyte reduced to one or a few cells?

4. Why does a Cycas tree with ovules need another tree nearby to set seed?