Question 2. When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Answer:
In the life cycle of a liverwort, reduction division takes place in the capsule for the formation of haploid spores that will grow into a gametophytic thallus after germination. In mosses, the gametes fertilise and a zygote is formed. The zygote develops into the sporophyte. In the capsule of the sporophyte, reduction division takes place, and haploid spores are formed, which on germination give rise to a gametophytic thallus. In ferns, reduction division takes place in the sporangia present on leaves for the production of spores. Gymnosperms, which have sporophytic plant bodies, show reduction division in their microsporangia and megasporangia for the formation of microspores and megaspores. In the case of angiosperms, the main plant body is diploid, and meiosis occurs for the formation of gametes. Therefore, in the life cycle of an angiosperm, reduction division takes place in the anther and ovary at the time of microspore and megaspore formation, respectively.
Question 3. Name the three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.
Answer:
Three groups of plants that have archegonia are given below:
- Bryophytes
- Pteridophytes
- Gymnosperms.
The life cycle of a pteridophyte then consists of two morphologically distinct phases:
(i) The gametophytic phase
(ii) The sporophytic phase.
These two phases come one after another in the life cycle of a pteridophyte, and this phenomenon is called alternation of generations. The gametophyte is a haploid with a single set of chromosomes. It produces male sex organs, antheridia and female sex organs, archegonia.
(i) The antheridia may be embedded or projecting types. Each antheridium has a single-layered sterile jacket enclosing a mass of astrocytes.
(ii) The astrocytes are flask-shaped, sessile or shortly stalked and differentiated into globular venters and tubular necks.
(iii) The diploid zygote is the first cell of the sporophytic generation. It is retained inside the archegonium and forms the embryo.
(iv) The antherozoids, after liberation from the antheridium, reach up to the archegonium, fuse with the egg and form a diploid structure known as a zygote.
(v) The archegonium contains a large egg, which is non-motile.
