Alternation of generations - Wikipedia
Source for information on Reproduction, Alternation of Generations and: Plant In many algae and fungi, in contrast, the diploid zygote undergoes meiosis. Request PDF on ResearchGate | Alternation of Generations in Plants and Algae | Photosynthetic organisms are found in most of the branches of the eukaryotic. What is a plant? Compare a typical plant and alga Alternation of Generations Lichen Mutualism. Campbell Fig. A,B fungal mycelium algal cells.
Gametophyte of Pellia epiphylla with sporophytes growing from the remains of archegonia. Antheridia and archegonia occur on the same gametophyte, which is then called monoicous. Many sources, including those concerned with bryophytes, use the term 'monoecious' for this situation and 'dioecious' for the opposite.
The liverwort Pellia epiphylla has the gametophyte as the dominant generation. The moss Mnium hornum has the gametophyte as the dominant generation. However, the parent sporophyte may be monoecious, producing both male and female gametophytes or dioecious, producing gametophytes of one gender only. Seed plant gametophytes are extremely reduced in size; the archegonium consists only of a small number of cells, and the entire male gametophyte may be represented by only two cells.
All spores the same size homospory or isospory. Horsetails species of Equisetum have spores which are all of the same size. When the two kinds of spore are produced in different kinds of sporangia, these are called megasporangia and microsporangia.
A megaspore often but not always develops at the expense of the other three cells resulting from meiosis, which abort. Megasporangia and microsporangia occur on the same sporophyte, which is then called monoecious. Most flowering plants fall into this category. Thus the flower of a lily contains six stamens the microsporangia which produce microspores which develop into pollen grains the microgametophytesand three fused carpels which produce integumented megasporangia ovules each of which produces a megaspore which develops inside the megasporangium to produce the megagametophyte.
In other plants, such as hazel, some flowers have only stamens, others only carpels, but the same plant i. Flowers of European Holly, a dioecious species: An individual tree of the European holly Ilex aquifolium produces either 'male' flowers which have only functional stamens microsporangia producing microspores which develop into pollen grains microgametophytes or 'female' flowers which have only functional carpels producing integumented megasporangia ovules that contain a megaspore that develops into a multicellular megagametophyte.
There are some correlations between these variations, but they are just that, correlations, and not absolute. For example, in flowering plants, microspores ultimately produce microgametes sperm and megaspores ultimately produce megagametes eggs. However, in ferns and their allies there are groups with undifferentiated spores but differentiated gametophytes.
Sexual life cycles
For example, the fern Ceratopteris thalictrioides has spores of only one kind, which vary continuously in size.
Smaller spores tend to germinate into gametophytes which produce only sperm-producing antheridia. The diagram shows the alternation of generations in a species which is heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious.
A seed plant example might be a willow tree most species of the genus Salix are dioecious. An immobile egg, contained in the archegonium, fuses with a mobile sperm, released from an antheridium. The resulting zygote is either 'male' or 'female'.
A 'male' zygote develops by mitosis into a microsporophyte, which at maturity produces one or more microsporangia. Microspores develop within the microsporangium by meiosis. In a willow like all seed plants the zygote first develops into an embryo microsporophyte within the ovule a megasporangium enclosed in one or more protective layers of tissue known as integument. At maturity, these structures become the seed. Later the seed is shed, germinates and grows into a mature tree.
A 'male' willow tree a microsporophyte produces flowers with only stamens, the anthers of which are the microsporangia. Microspores germinate producing microgametophytes; at maturity one or more antheridia are produced.
Sperm develop within the antheridia. In a willow, microspores are not liberated from the anther the microsporangiumbut develop into pollen grains microgametophytes within it. The whole pollen grain is moved e. A 'female' zygote develops by mitosis into a megasporophyte, which at maturity produces one or more megasporangia.
Megaspores develop within the megasporangium; typically one of the four spores produced by meiosis gains bulk at the expense of the remaining three, which disappear. Megaspores germinate producing megagametophytes; at maturity one or more archegonia are produced. Eggs develop within the archegonia.
The carpels of a willow produce ovules, megasporangia enclosed in integuments. Within each ovule, a megaspore develops by mitosis into a megagametophyte. An archegonium develops within the megagametophyte and produces an egg.
The whole of the gametophytic 'generation' remains within the protection of the sporophyte except for pollen grains which have been reduced to just three cells contained within the microspore wall. Life cycles of different plant groups[ edit ] The term "plants" is taken here to mean the Archaeplastidai.
Alternation of generations occurs in almost all multicellular red and green algae, both freshwater forms such as Cladophora and seaweeds such as Ulva. In most, the generations are homomorphic isomorphic and free-living.
Some species of red algae have a complex triphasic alternation of generations, in which there is a gametophyte phase and two distinct sporophyte phases. For further information, see Red algae: Land plants all have heteromorphic anisomorphic alternation of generations, in which the sporophyte and gametophyte are distinctly different.
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As an illustration, consider a monoicous moss. Antheridia and archegonia develop on the mature plant the gametophyte. In the presence of water, the biflagellate sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte.
The sporophyte grows up from the archegonium. Its body comprises a long stalk topped by a capsule within which spore-producing cells undergo meiosis to form haploid spores.
Most mosses rely on the wind to disperse these spores, although Splachnum sphaericum is entomophilousrecruiting insects to disperse its spores. For further information, see Liverwort: Life cycleMoss: Life cycleHornwort: Diagram of alternation of generations in liverworts.
Moss life cycle diagram Hornwort life cycle diagram In ferns and their allies, including clubmosses and horsetailsthe conspicuous plant observed in the field is the diploid sporophyte. The haploid spores develop in sori on the underside of the fronds and are dispersed by the wind or in some cases, by floating on water. If conditions are right, a spore will germinate and grow into a rather inconspicuous plant body called a prothallus.
The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations.
The prothallus is short-lived, but carries out sexual reproduction, producing the diploid zygote that then grows out of the prothallus as the sporophyte. For further information, see Fern: Diagram of alternation of generations in ferns. A gametophyte prothallus of Dicksonia sp. A sporophyte of Dicksonia antarctica. The underside of a Dicksonia antarctica frond showing the sori, or spore-producing structures. In the spermatophytesthe seed plants, the sporophyte is the dominant multicellular phase; the gametophytes are strongly reduced in size and very different in morphology.
In many algae and fungi, in contrast, the diploid zygote undergoes meiosis immediately to form haploid cells, called spores. Spores subsequently grow into multicellular haploid individuals. In both of these life cycles there is only one multicellular phase. In some algae and in all plants, however, there are actually two multicellular phases, one haploid and one diploid, which alternate with each other in the life cycle.
This type of reproductive cycle is referred to as alternation of generations. In organisms with alternation of generations, the diploid generation, or sporophyte, is formed by mitotic divisions of the diploid zygote, just as in animals. When mature, the sporophyte produces asexual reproductive organs called sporangia. Meiosis within the sporangia produces the one-celled, haploid spores that are released when the sporangia open.
Each spore then gives rise to a multicellular haploid individual, or gametophyte. The game-tophyte produces the sexual reproductive organs, or gametangia, in which haploid gametes are formed by mitosis. Gametes then fuse to form the zygote, completing the cycle. Occasionally, sporophyte and gametophyte generations look identical, as in many red and some green and brown algae, in which case alternation of generations is described as isomorphic.
In other algae and all plants, the two generations are structurally different, and alternation of generations is said to be heteromorphic.
It is notable that isomorphic alternation of generations occurs only in certain algae and aquatic molds, while heteromorphic alternation of generations is the rule in land plants.Alternation of Generations Life Cycle
In bryophytes the gametophyte is the ecologically persistent, independent generation, and the sporophyte is ephemeral and dependent upon the gametophyte for its nutrition.
In all other plants the sporophyte dominates the life cycle.