cycles de développement

Life Cycles in Plants and it Types
(With Diagram)
Article Shared by Trisha
The life cycles of all
sexually reproducing
plants follow a pattern of
alternation between a
haploid, sexual generation
called gametophyte with a
diploid, asexual generation
called sporophyte.
This phenomenon of
alternation between
gametophyte (n) and
sporophyte (2n) in the life
cycle of a plant is called
alternation of generations
(Hofmeister, 1851).
Alternation of generations
is of two types- isomorphic
and heteromorphy. In
isomorphic (or
homologous) type both the
alternating generations are
morphologically similar,
while in heteromophic (or
heterologous) type of
alternation of generations
both the generations of life
cycle are morphologically
dissimilar.
In the life cycle, change of ploidy level from diploid (2n) to haploid (n) is brought about by meiosis and
fertilization (syngamy) respectively. The sporophyte generation is “diploid” in which meiosis occurs to
produce haploid spores, while gametophyte generation is “haploid” in which gametes are produced (by
mitosis of haploid cells).
In the life cycle, any one of the two generations is found to be always more conspicuous and survive a
greater proportion is called as dominant generation. In algae and bryophytes the somatic or vegetative plant
body is a gametophyte which is dominant and visible. But in pteridophytes, gymnosperms and angiosperms
the visible plant body is a sporophyte which is dominant and the gametophyte generation is less conspicuous
(Fig. 4.2).
In the life cycle diagram, the dominant generation should be placed in the top half of the cycle. Plant life
cycles differ from animal life cycles by adding a haploid gametophyte generation after meiosis and before
the production of gametes.

Types of Plant Life Cycles (Fig. 4.1):
In the life cycles of plants one or more morphologically distinct generations appear.
On this basis, life cycle may be three types:
(i) Uniphasic:
In this life cycle, only one visible generation is distinct either haploid or diploid and the life cycle are called
haplontic, or diplontic.
(ii) Diphasic:
In this type, both the haploid and diploid generations are distinct. The life cycle is called haplo-diplontic or
diplo-haplontic
(iii) Triphasic:
In this case, three dissimilar generations are distinct, and accordingly, life cycle may be haplo-haplo-
haplontic or diplo-diplo-haplontic type. Broadly speaking plant life cycles are of five types i.e. haplontic,
diplontic, haplo-diplontic, haplobiontic (= haplo-haplontic) and diplobiontic (= diplo-diplo-haplontic).
(a) Haplontic life cycle:
It is the primitive type of life cycle characterized by dominant gametophyte and zygotic meiosis. Here, free-
living sporophyte absent. The sporophyte generation is represented only by the one-celled zygote,
sometimes called zygospore. The zygote undergoes meiosis to form haploid spores. Each spore germinates
(divide mitotically) to form gametophyte. Haplontic life cycle found in many algae like Volvox, Spirogyra,
Ulothrix, Chlamydomonas etc.
(b) Diplontic life cycle:
The life cycle characterized by gametic meiosis and diploid sporophyte this is dominant, photosynthetic and
independent generation of the plant. The gametophyte generation is represented by the haploid gametes or
few celled haploid gametophyte. Here only sporophyte is conspicuous and gametophyte is inconspicuous.
Diplontic life cycle is exhibited by some green algae, brown algae and all seed bearing plants i.e.
gymnosperms and angiosperms. In seeded plants pollen and ovaries contain male and female gametophytes,
respectively.

(c) Haplo-diplontic life cycle
This type of life cycle involves the alternation of two vegetative individuals, the haploid gametophyte and
diploid sporophyte. In this case sporogenic meiosis occurs in sporophyte to produce spores (meiospores).
Here both the generations are conspicuous. This type of life cycle is exhibited by some green algae, brown
algae, bryophytes and pteridophytes.
(d) Haplobiontic (= haplo-haplontic) life cycle:
It is also called diphasic-haplobiontic, where two successive vegetative phases are gametophytes that
alternate with a diploid short-lived sporophyte, e.g. red algae like Nemalion, etc.
(e) Diplobiontic (= diplo-diplo-haplontic):
It is also called diphasic-diplobiontic, where two successive diploid phases alternate with the gametophyte.
The following points highlight the three types of life cycle of plant groups. The types are: 1. Haplontic
2. Diplontic 3. Haplodiplontic.
Type # 1. Haplontic (Fig. 3.24):
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 can
multiply vegetatively and by accessory spores or mitospores.
Ultimately 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 (e.g., Spirogyra,
Zygnema, Vaucheria, etc.). In others the protoplast of the zygote cleaves into four meiospores (zoospores or
aplanospores). 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. Some authors consider that there is an incipient alternation of generations because the zygote
behaves as an incipient sporophyte by producing 4—16 meiospores.
Examples are found in Chlamydomonas, Volvox Spirogyra, Ulothrix, Oedogonium, Chara, Coleochaete and
several other chlorophyceae, xanthophyceae and some members of other groups.
Fritsch (1935, 1942) and Stebbins (1960) consider the haplontic life history to be the most primitive.
However, Feldmann (1952) believes that the haplontic life history has been derived in nature by the
elimination of diploid somatic phase in a diplohaplontic life history.

Type # 2. Diplontic (Fig. 3.25):
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.
Examples:
Occur in Cladophora glomerata, Caulerapa, Bryopsis, Codium and many other siphonales, some
chlorococcales and fucales like Fucus and Sargassum. The diplontic life history of fucales is clearly derived
from a diplohaplontic life history where the gametophytic or haploid somatic phase gets eliminated through
progressive evolution.
In gymnosperms and angiosperms, sporophytic generation is dominant and independent while
gametogenetic generation is highly reduced and is dependent. Therefore, some authors call their life cycle to
be diplontic though the same is reduced diplohaplontic.
Type # 3. Haplodiplontic (Fig. 3.26):
This type of life history involves the sequential recur-
rence 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 de-
velops 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, e.g., Dictyota, bryophytes, pteridophytes.