describe the alternation of generation in angiosperms with diagram
Answers
Answer:
Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether or not these forms are free-living. ... The sporophyte produces free-swimming haploid spores by meiosis that germinate into haploid gametophytes.
Explanation:
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Answer: The alternation of generations is an important concept in the evolution of plants. All land plants have alternation of generations.
In mosses and their relatives (Bryophytes), the haploid gametophyte is the dominant generation, and the diploid sporophytes are sporangium-bearing stalks growing from the gametophytes. In ferns, the diploid sporophyte is much larger, but the haploid gametophyte is also a little plant that can grow for a long time.
For flowering plants (Angiosperms), the sporophyte generation is almost the whole life cycle (the green plant, roots etc.) except the small reproductive structures (pollen and ovule).
The sporophyte produces spores (hence the name), by meiosis. These develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they have half as many chromosomes. Later, the mature gametophyte produces male or female gametes (or both) by mitosis. The fusion of male and female gametes (fertilization) produces a diploid zygote which develops into a new sporophyte. This is the cycle which is known as alternation of generations or alternation of phases.
As a factor in plant evolution
In the landmark work Variation and evolution in plants, Stebbins discussed how alternation of generations related to the overall evolution of plants. He began:
"The most striking difference between the sexual cycle of animals and those found in plants is that, with the exception of a few Protozoa, animals are diploid at all stages, while nearly all plants possess a haploid stage of greater or lesser duration. Furthermore, the sequence of types of alternation of generations... is one of the best-known features of plant evolution... The diploid generation has undoubtedly evolved independently many different times".
Later Stebbins comments:
"The diploid condition brings about an increase in flexibility because it makes possible the condition of genetic dominance and recessiveness. In a haploid organism every new mutation is immediately exposed to the action of selection... In a diploid organism, on the other hand, each new mutant arises as a heterozygote and, if recessive, is sheltered from selection".
The point is that, in diploids, new alleles are sheltered and (collectively) they are a reservoir of potential variation in the population.
Algae
Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar (isomorphic).
Bryophytes
Bryophytes (mosses, liverworts and hornworts) have a dominant gametophyte stage on which the adult sporophyte is dependent on the gametophyte for nutrition. The sporophyte develops from the zygote inside the female sex organ, so its early development is nurtured by the gametophyte.
Vascular plants
In flowering plants, the sporophyte comprises the whole multicellular body except the pollen and megagametophyte within the ovule
An independent sporophyte is the dominant form in all clubmosses, horsetails, ferns, gymnosperms, and angiosperms (flowering plants) that have survived to the present day.
Earlier evolution
Early land plants had sporophytes that produced identical spores: they looked the same whichever sex they developed into. The ancestors of the gymnosperms evolved complex heterosporous life cycles: the spores producing male and female gametophytes were of different sizes. The female megaspores tending to be larger, and fewer in number, than the male microspores.
During the Devonian, several plant groups independently evolved heterospory and later endospory, in which single megaspores were kept inside the sporangia of the parent sporophyte. These endosporic megaspores had a miniature multicellular female gametophyte with female sex organs and egg cells. The ova were fertilised by free-swimming sperm produced by windborne miniatuarised male gametophytes in the form of pre-pollen.
The resulting zygote developed into the next sporophyte generation while still inside the single large female megaspore in the sporangium of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms.
Similar processes in other organisms
Some Chromalveolata, some fungi and some slime moulds have what seems to be genuine alternation of generations. These groups include such a wide range of different types that lt is difficult to say how common the phenomenon is. Certainly slime moulds are not a monophyletic group, and that may be true of the other two groups,
