life-cycle patterns are found in algae. However, there is no regular
and fixed alternation of generation, as found in higher palnts. In
blue-green algae and certain chlorophyceae which reproduced asexually,
there is n alternation generation.
type : This is a simplest and most primitive type of life-cycle.
The other pattern of life-cycle have originated from this type. This
type is found in all chlorophyceae.
cases the somatic phasse (plant) is haploid (Gametophyte) while the
diploid phase (sporophte) is represented by zygote. During germination
the zygote (2n) divides meiotically producing haploid (n) zoospores,
which develop into individual plant. Here the unicellular or filamentous
gametophyte (n) alternates with one-celled zygote or sporophyte (2n).
The haploid filamentous plants are known as haplonts which reproduce
asexually by zoospores or aplanospores producing the individals like
type: This pattern is reverse of haplontic type. In this case
somatic phase (plant) is diploid (sporophyte 2n) while the haploid
phase (gametophyte) is restricted to gmetes which are produced by
meiotic division. After gametic union a diploid zygote is formed,
which develops into a diploid (sporophyte 2n) plant by mitotic divisions.
type: In this type there are two exactly similar somatic phases
(morphologically identical plants) showing alernaion of generations.
Here the one phase is diploid (sporophyte 2n) while the other haploid
(gametophyte n). This pattern of life cycle is found in ulvaceae,
chaetophoraceae, cladophoraceae in chlorophyceae and ectocarpales,
dictyotaes of phaeophyceae.
cases, the zygote develops into a diploid multicellular plant (sporophyte)
by postponement of meiosis. Prior to zoospore (meiospore) formation
thee is meiosis. These zoospores (n) develop into haploid plants (gametophyte
n). The haploid plants prouce gametes (n) which after fusion develop
int zygote (2n)
type : This pattern of life cycle is exactly like that of the
preceding one (isomorphic type) only with the difference that the
alternating haploid (n) and diploid (2n) somatic phases (plant) and
morphologically different. In such cases the diploid multicellular
sporophytic plants produces haploid zoospores (meiospores) by meiosis.
these zoospores develop into gameophytes. eah gametophytic plant (n)
produce gametes which after their union form a zygote and the latter
develops into a diploid sporophytic plant by mitotic division. this
pattern is found in laminariales of phaeophyceae.
type : In this patern there are three phases in the life cycle.
Out of three phases two phases are haploid (n) and one diploid (2n).
A haploid gametophytic phase produces gametess which on fusion form
a zygote (2n). The latte is the only diploid phase which divides meiotically
forming a haploid asexual phase known as carposporophyte. The latter
reproduce asexually by haploid carpospores which again develop into
gametophytes (haploid plants n). Thus two morphologically different
haploid phases (gametophyte and carposporophyte) alternate with the
type : This type of life cycle is found in almost all Rhodophyceae
except Nemalionales. Here the life cycle is triphasic and involves
an alternation of two diploid (2x) or sporophytic generations. i.e.
carposporophyte and tetrasporophyte with one haploid (x) or gametophytic
generation. Thus there are two diploid phases and one haploid phase.
the gametophyte produce gametes which unite and orm a zygote (2x).
Now the zygote divides mitotically forming a carposporophyte (2x)
bearing diploid (2x) carpospores. On germination these diploid carpspores
form another diploid plant, the tetrasporophyte. The latter produce
tetraspores by meiosis. The haploid tetraspores erminate and give
rise to haploid gametophytic plants.
there are several pattern of life cycle in algae and there is no regular
and fixed alternation of generations as found in higher plants
cycle of Ulva
cycle of Sargassum
cycle of Porphyra