Exercises · 3.8
Q.Explain briefly the following terms with suitable examples:-
(i) protonema
(ii) antheridium
(iii) archegonium
(iv) diplontic
(v) sporophyll
(vi) isogamy
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Start your 14-day free trial to unlock the full solution →Each of these terms describes a reproductive stage or structure in plants — the young moss thread, the male and female sex organs, a life cycle ruled by the sporophyte, a spore-bearing leaf, and the fusion of identical gametes.
- Protonema. When a moss spore germinates, the very first thing it forms is not a leafy plant but a protonema. This is a creeping, green, branched structure that is often filamentous, resembling a mat of fine green threads. It is the first of the two gametophyte stages of a moss; the leafy stage arises later as a lateral bud on it. Funaria is a familiar moss that shows this stage.
- Antheridium. This is the male sex organ. In bryophytes and pteridophytes the sex organs are multicellular, and the antheridium is the structure that produces the male gametes. In bryophytes these gametes are the antherozoids, each carrying two flagella so that it can swim through a film of water to reach the egg. A moss such as Funaria bears antheridia at the tips of its leafy shoots.
- Archegonium. This is the female sex organ, a flask-shaped, multicellular structure that holds a single egg. The swimming male gamete travels to the mouth of the archegonium and fuses with the egg inside it to form the zygote. It is found in bryophytes, pteridophytes and gymnosperms; a liverwort such as Marchantia is a good example of a plant that bears archegonia.
- Diplontic. This describes a life cycle in which the diploid sporophyte is the dominant, conspicuous phase and the gametophyte is much reduced. The seed plants illustrate this pattern well: in gymnosperms and angiosperms the plant we see is the sporophyte, and the gametophytes are tiny and never live independently.
Note
As one moves from bryophytes to pteridophytes to the seed plants, the sporophyte becomes steadily more dominant. The diplontic condition is the endpoint of that trend. …
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