Bryophytes and Pteridophytes: Quick Revision

A brief note on Bryophytes and Pteridophytes. Read it before a test. Try each question at the end before you open its answer.

Bryophytes

• The amphibians of the plant kingdom: they live on land but need water for fertilisation.
• The main plant body is the haploid gametophyte, held by rhizoids. No true roots, stems or leaves; no vascular tissue.
• Antheridia make biflagellate antherozoids; archegonia (flask-shaped) make one egg.
• The zygote grows into a sporophyte (foot, seta, capsule) that depends on the gametophyte. Spores form by meiosis.
• Liverworts: flat thalli such as Marchantia; asexual reproduction by gemmae in gemma cups.
• Mosses: a protonema grows from the spore, then a leafy stage with branched rhizoids. Funaria, Polytrichum, Sphagnum.
• Uses: food for animals, peat from Sphagnum, first settlers on rock, protection against soil erosion.

Pteridophytes

• The first land plants with xylem and phloem.
• The main plant body is the diploid sporophyte, with true roots, stems and leaves.
• Sporangia on sporophylls; in some, compact cones called strobili (Selaginella, Equisetum).
• A spore grows into a small, free-living prothallus with antheridia and archegonia. Water is needed for fertilisation.
• Classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), Pteropsida (Dryopteris, Pteris, Adiantum).

Heterospory

• Most pteridophytes are homosporous. Selaginella and Salvinia are heterosporous.
• Megaspores grow into female gametophytes; microspores into male ones.
• The female gametophyte stays on the parent, and the embryo starts to grow inside it: a forerunner of the seed habit.

A worked example

A fern frond has $12$ pinnae with $5$ sori each. Each sorus holds $40$ sporangia, and each sporangium has $16$ spore mother cells. How many spores does the frond release?

1. Sporangia. $12 \times 5 \times 40 = 2400$ sporangia.
2. Spores in one sporangium. $16 \times 4 = 64$, because meiosis gives $4$ spores from each mother cell.
3. All the spores. $2400 \times 64 = 153\,600$ spores.

Each spore is haploid, and each can grow into a prothallus, if it lands in a cool, damp, shady place.

Practice with new numbers

A program to try

The program sorts four plants with a two-question key.

Mistakes to avoid

• The fern plant is not the gametophyte. It is the diploid sporophyte; the prothallus is the gametophyte.
• Rhizoids are not roots. True roots appear first in pteridophytes.
• The moss sporophyte is not free-living. It feeds on the gametophyte.
• Spores are haploid. They are made by meiosis.
• Selaginella makes no seeds. Heterospory is only a step towards the seed habit.

Check yourself

1. Why are bryophytes called the amphibians of the plant kingdom?

2. Name the gametophyte of a fern, and say whether it is haploid or diploid.

3. Into which class would you put the club moss, Lycopodium?

4. What happens inside the female gametophyte of Selaginella that makes it a step towards seeds?

5. Give one way in which mosses help to form soil on bare rock.