ALTERNATION OF GENERATION
- Different plant groups complete their life cycles in different patterns. The phases in which the life cycle is completed follow each
other rigidly. This is called alternation of generations. - Life cycle may be of three types: haplontic, diplontic and haplodiplontic.
- There is a single vegetative individual or somatic phase. It is haploid and
is often called gametophyte. The haploid plant body may be unicellular,
colonial or multicellular. - It gives rise to haploid gametes. The gametes fuse and produce a diploid
zygote. The zygote remains single-celled. It does not multiply itself, neither
does it give rise to a multicellular diploid structure. Instead it may take some
rest. Meiosis occurs at the time of zygote germination. Four haploid nuclei
are formed as a result. Three of them degenerate in some cases and the
haploid protoplast of the zygote gives rise to new plant. - In others the protoplast of the zygote cleaves into four meiospores.
- The latter may divide further into 8-16 spores before liberation. An alternation of generations is absent since the plant does not
have two cytologically distinct somatic phases. - Haplontic life cycle occurs in many algae such as Volvox, Spirogyra and Chlamydomonas, etc.
- There is a single somatic phase or vegetative individual. It is diploid and is often called sporophyte though it produces gametes
in its body or sex organs. The diploid plant body is elaborated by the growth of the diploid zygote. It may multiply vegetatively
and by producing accessory spores. Meiosis occurs in the plant body or its sex organs at the time of gamete formation. Therefore,
the gametes are the only haploid structures in the life. They fuse during fertilisation and give rise to the diploid individual of the
progeny. Alternation of generations is absent in diplontic life history. - Diplontic life cycle occurs in Fucus, Cladophora, Cauterapa, etc.
- This type of life history involves the sequential recurrence of two well developed somatic phases or vegetative individuals,
gametophyte and sporophyte. The sporophyte possesses diploid chromosome number (2n). Meiosis takes place in it at the time
of formation of meiospores. The haploid meiospores germinate to produce haploid gametophytes. The gametophytes produce
gametes. The fusion product of gametes is a diploid zygote which develops into the sporophytic thallus of the progeny. There
is thus a clear alternation of generations between a haploid gamete producing gametophyte and a diploid spore producing
sporophyte in diplohaplontic life history. - Haplodiplontic life cycle occurs in bryophytes, pteridophytes and some algae such as Dictyota.
- In which of the following plants, stem is jointed and ribbed
with leaves and branches are borne in whorls?
(a) Equisetum (b) Selaginella
(c) Lycopodium (d) Cooksonia - Circinate vernation in ferns refers to
(a) specialised pattern of leaf development in which leaf
unrolls from the fiddle head
(b) system of leaf gaps in the stem
(c) acropetally arranged fronds
(d) presence of sori on the leaf surface.
3. Select the mismatched pair.
(a) Selaginella - Club moss
(b) Dryopteris - Maiden hair fern
(c) Ophioglossum - Adders tongue fern
(d) Equisetum - Horsetail fern
4. In which of the following options, do all listed genera belong
to the same class of pteridophytes?
(a) Lycopodium, Psilotum, Selaginella, Rhynia
(b) Rhynia, Cooksonia, Psilotum, Tmesipteris
(c) Selaginella, Lycopodium, Equisetum, Dryopteris
(d) Psilotum, Adiantum, Lycopodium, Equisetum
- In which of the following plants, stem is jointed and ribbed