Life cycle of Adiantum
Life Cycle of Adiantum
Adiantum is sporophyte, bearing asexual reproductive organs called
sporangia, arranged in groups called sori. The sori occur on the under surface
of the margins of pinnules. The sporangium is a flattened, biconvex capsule
borne on a multicellular stalk. The capsule contains a mass of small asexual
reproductive bodies called spores. Each spore is a simple cell, consisting
of cytoplasm and nucleus, surrounded by two-layered cell wall. The inner
Alternation of generation
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young sporophyte Life cycle of Adiantum
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called intine or endosporium, while the outer thick
cutenised layer is called exine or exosporium. The spores are small and
light, and are liberated from the sporangia in dry weather. When a spore falls
on a suitable soil, it begins to germinate in about a week's time and produces
a haploid gametophyte called prothallus.
The prothallus is green, small, flat and heart shaped structure. From
the under surface of the prothallus arise a number of long, brown rhizoids
which pass down into the soil. The rihzoids fix the prothallus to the soil and
absorb water and mineral salts from the soil. The prothallus manufactures its
own food with the help of its chloroplasts. Thus the prothallus is an
independent plant. The prothallus is hermaphrodite, bearing both the
antheridia and archegonia on its under surface. Each antheridium is a
spherical capsule which contains many small cells called sperm mother
cells or spermatocytes. Each spermatocyte gives rise to a male gamete or
sperm. Each sperm is spirally coiled body with two tuft of fine cilia at one
end. Each archegonium is a flask shaped body consisting of a tube like neck
and a basal swollen venter. The venter contains a single long naked cell
called egg or oosphere.
Fertilization occurs when the soil is thoroughly wet with water. A
number of sperms cluster round the open mouth of the archegonium. Finally
one of them enter the archegonium and passes down to the venter. Here its
nucleus fuses with that of the oosphere to form oospore or zygote. The
fertilized oosphere becomes oospore by secreting a wall around itself. Soon
after fertilization the cospore begins to divide to produce an embryo which
develops into a young sporophyte of Adiantum.
Alternation of Generation in Adiantum
The life history of Adiantum includes two quite distinct generations,
the sporophyte generation and the gametophyte generation. The sporophyte
produce spores which on germination gives rise to a heart -
shaped
gametophyte or prothallus. The gametophyte develops antheridia and
archegonia, which contain sperms and eggs respectively. The oospore
resulting from the fusion of the sperm with the egg does not give rise to
gametophyte but grow into an independent young sporophyte of Adiantum.
Thus the sporophyte gives rise to the gametophyte and the gametophyte to
the sporophyte is known as alternation of generation.
1. They are dominant plants of the world next to spermatophytes and have
colonized major shady areas near water course and moist places.
2.
Importance of seedless vascular plants
importance as they represent connecting link between Bryophytes and
Psilopsida, agroup of seedless vascular plants have evolutionary
pteidophytes.
They have special role in enriching the flora of the earth especially of
the tropics.
4. Some of the ferns like tmisipitaris, pteridium etc are edible and their
young shoots are used as vegetables.
5. Seedless vascular plants invade the habitats as a transitional community
during the process of plant succession and hence perform a major role in the
establishment of plant communities. of plait t have
community bata hai is
co
Seed Plants
Evolution of Seed
Seed may be defined as a ripened and fertilized ovule. The
evolution of seed is the most important process in the evolution of vascular
plants because seed is the structure which can survive under unfavorable
conditions. It is responsible for the preservation of species. Seeds have
made the plants dominant vegetation on land.
The evolution of seed involve the following steps:
a. Development of heterospory o produce typed a spoves
All the seed plants are heterosporous. They produce two types of
spores namely microspores and megaspores. The microspores produced in
the microsporangia (pollen sac) germinate into male gametophyte and
megaspore produced in the megasporangia (ovules) germinate into female
gametophyte (embryo sac).
B. Retention of megaspore inside the sporangium
Instead of being shed from the sporangium like the spores of the
lower plants, the megaspores of the seed plants are permanently embedded
and protected inside the megasporangium. Here the megaspore develops
into a small female gametophytes.
Microspore
Megaspore Megaspore
Female
gametophyte
Nucellus
Integument
Reduction to
Sporangium
one megaspore
enveloped by
Heterospory and endospory
integuments
Main steps in the evolution of seed
3. They have special role in enriching the flora of the earth especially of
the tropics.
4. Some of the ferns like tmisipitaris, pteridium etc are edible and their
young shoots are used as vegetables.
5. Seedless vascular plants invade the habitats as a transitional community
during the process of plant succession and hence perform a major role in the
establishment of plant communities. of plait t have
community bata hai is
co
Seed Plants
Evolution of Seed
Seed may be defined as a ripened and fertilized ovule. The
evolution of seed is the most important process in the evolution of vascular
plants because seed is the structure which can survive under unfavorable
conditions. It is responsible for the preservation of species. Seeds have
made the plants dominant vegetation on land.
The evolution of seed involve the following steps:
a. Development of heterospory o produce typed a spoves
All the seed plants are heterosporous. They produce two types of
spores namely microspores and megaspores. The microspores produced in
the microsporangia (pollen sac) germinate into male gametophyte and
megaspore produced in the megasporangia (ovules) germinate into female
gametophyte (embryo sac).
B. Retention of megaspore inside the sporangium
Instead of being shed from the sporangium like the spores of the
lower plants, the megaspores of the seed plants are permanently embedded
and protected inside the megasporangium. Here the megaspore develops
into a small female gametophytes.
Microspore
Megaspore Megaspore
Female
gametophyte
Nucellus
Integument
Reduction to
Sporangium
one megaspore
enveloped by
Heterospory and endospory
integuments
Main steps in the evolution of seed
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c. Formation of integument around the megasporangium and evolution of ovule
The fossil record shows that there were certain fern like plants that
bore seeds. In this case the megasporangium was surrounded by branch like
outgrowths. These outgrowths became fused, during evolution, to form an
envelope like structure called integuments, around the sporangium. The
integuments form protective covering. The integumented
megasporangium in which the megaspore is retained is called an ovule or
unripe seed. The ovule also contains large quantity d for the
developing embryo. These two characters protection and are the best
adaptive characters of land plants to their environment to make them
prominent on earth.
d. Evolution of pollen tube
The non vascular plants and the lower vascular plants require water
for fertilization. They produce motile flagellated sperms that reach the non
motile egg through water from rain or dew etc. However the seed plants do
not depend on water for fertilization. The evolution of pollen tube parallels
the evolution of seed.
The egg produced in the ovule is enclosed and protected by the
integuments and the sperms would not be able to reach the egg. This
obstacle is overcome by the development of pollen tube that acts as a
vehicle for the transport of sperms to the eggs inside the ovule ensure
fertilization. Thus the seeded plants can grow in a variety of terrestrial
environments from the alpine environments of the mountains to the arid
regions of the desert.
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