Editing Alternation of generations (section)

==Alternation of generations in plants==

===Fundamental elements===

The diagram above shows the fundamental elements of the alternation of generations in plants. There are many variations in different groups of plants. The processes involved are as follows:<ref name="Foster-1974">{{Harvnb|Foster|Gifford|1974}},{{pn|date=May 2023}} {{Harvnb|Sporne|1974a}}{{pn|date=May 2023}} and {{Harvnb|Sporne|1974b}}. {{pn|date=May 2023}} </ref>
* Two single-celled haploid gametes, each containing ''n'' unpaired chromosomes, fuse to form a single-celled diploid zygote, which now contains ''n'' pairs of chromosomes, i.e. 2''n'' chromosomes in total.<ref name="Foster-1974"/>
* The single-celled diploid zygote germinates, dividing by the normal process ([[mitosis]]), which maintains the number of chromosomes at 2''n''. The result is a multi-cellular diploid organism, called the ''sporophyte'' (because at maturity it produces spores).<ref name="Foster-1974"/>
* When it reaches maturity, the sporophyte produces one or more '''sporangia''' (singular: sporangium) which are the organs that produce diploid spore mother cells (sporocytes). These divide by a special process ([[meiosis]]) that reduces the number of chromosomes by a half. This initially results in four single-celled haploid spores, each containing ''n'' unpaired chromosomes.<ref name="Foster-1974"/>
* The single-celled haploid spore germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at ''n''. The result is a multi-cellular haploid organism, called the ''gametophyte'' (because it produces gametes at maturity).<ref name="Foster-1974"/>
* When it reaches maturity, the gametophyte produces one or more '''gametangia''' (singular: gametangium) which are the organs that produce haploid gametes. At least one kind of gamete possesses some mechanism for reaching another gamete in order to fuse with it.<ref name="Foster-1974"/>
The 'alternation of generations' in the life cycle is thus between a diploid (2''n'') generation of multicellular sporophytes and a haploid (''n'') generation of multicellular gametophytes.<ref name="Foster-1974"/>

[[File:Onoclea sensibilis 4 crop.jpg|thumb|Gametophyte of the fern ''[[Onoclea sensibilis]]'' (flat thallus, bottom) with a descendant sporophyte beginning to grow from it (small frond, top) ]]

The situation is quite different from that in animals, where the fundamental process is that a multicellular diploid (2''n'') individual {{em|directly}} produces haploid (''n'') gametes by meiosis. In animals, spores (i.e. haploid cells which are able to undergo mitosis) are not produced, so there is no asexual multicellular generation. Some insects have haploid males that develop from unfertilized eggs, but the females are all diploid.<ref name="Foster-1974"/>

=== Variations ===

The diagram shown above is a good representation of the life cycle of some multi-cellular algae (e.g. the genus ''[[Cladophora]]'') which have sporophytes and gametophytes of almost identical appearance and which do not have different kinds of spores or gametes.<ref name="Guiry-2008">{{Harvnb|Guiry|Guiry|2008}}</ref>

However, there are many possible variations on the fundamental elements of a life cycle which has alternation of generations. Each variation may occur separately or in combination, resulting in a bewildering variety of life cycles. The terms used by botanists in describing these life cycles can be equally bewildering. As Bateman and Dimichele say "[...] the alternation of generations has become a terminological morass; often, one term represents several concepts or one concept is represented by several terms."<ref>{{Harvnb|Bateman|Dimichele|1994|p=347}}</ref>

Possible variations are:
* ''Relative importance of the sporophyte and the gametophyte.''
** ''Equal'' ('''homomorphy''' or '''isomorphy''').<br />Filamentous algae of the genus ''[[Cladophora]]'', which are predominantly found in fresh water, have diploid sporophytes and haploid gametophytes which are externally indistinguishable.<ref name="Shyam-1980">{{Harvnb|Shyam|1980}}</ref> No living land plant has equally dominant sporophytes and gametophytes, although some theories of the evolution of alternation of generations suggest that ancestral land plants did.
** ''Unequal'' ('''heteromorphy''' or '''anisomorphy''').[[Image:Mnium hornum 2005.04.02 14.55.41.jpg|thumb|Gametophyte of ''[[Mnium hornum]]'', a moss ]]
*** ''Dominant gametophyte'' ('''gametophytic''').<br />In liverworts, mosses and hornworts, the dominant form is the haploid gametophyte. The diploid sporophyte is not capable of an independent existence, gaining most of its nutrition from the parent gametophyte, and having no chlorophyll when mature.<ref>{{Harvnb|Watson|1981|p=2}}</ref>[[Image:Blechnum discolor - crown fern.jpg|thumb|Sporophyte of ''[[Lomaria discolor]]'', a fern ]]
*** ''Dominant sporophyte'' ('''sporophytic''').<br />In ferns, both the sporophyte and the gametophyte are capable of living independently, but the dominant form is the diploid sporophyte. The haploid gametophyte is much smaller and simpler in structure. In seed plants, the [[Gametophyte#Seed plants|gametophyte]] is even more reduced (at the minimum to only three cells), gaining all its nutrition from the sporophyte. The extreme reduction in the size of the gametophyte and its retention within the sporophyte means that when applied to seed plants the term 'alternation of generations' is somewhat misleading: "[s]porophyte and gametophyte effectively function as a single organism".<ref name="Bateman-1994-2"/> Some authors have preferred the term 'alternation of phases'.<ref name="StewartRothwell-1993"/>
* ''Differentiation of the gametes.''
** ''Both gametes the same'' ('''isogamy''').<br />Like other species of ''[[Cladophora]]'', ''C. callicoma'' has flagellated gametes which are identical in appearance and ability to move.<ref name="Shyam-1980" />
** ''Gametes of two distinct sizes'' ('''anisogamy''').
*** ''Both of similar motility''.<br />Species of ''[[sea lettuce|Ulva]]'', the sea lettuce, have gametes which all have two flagella and so are motile. However they are of two sizes: larger 'female' gametes and smaller 'male' gametes.<ref name="Kirby-2001">{{Harvnb|Kirby|2001}}</ref>
*** ''One large and sessile, one small and motile'' ('''oogamy'''). The larger sessile megagametes are eggs (ova), and smaller motile [[microgamete]]s are sperm (spermatozoa, spermatozoids). The degree of motility of the sperm may be very limited (as in the case of flowering plants) but all are able to move towards the sessile eggs. When (as is almost always the case) the sperm and eggs are produced in different kinds of gametangia, the sperm-producing ones are called '''antheridia''' (singular antheridium) and the egg-producing ones '''archegonia''' (singular archegonium).[[Image:Pellia epiphylla IMG 1610.jpg|thumb|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.<ref>{{Harvnb|Watson|1981|p=33}}</ref><ref>{{Harvnb|Bell|Hemsley|2000|p=104}}</ref> Here 'monoecious' and 'dioecious' are used only for sporophytes.)<br />The liverwort ''Pellia epiphylla'' has the gametophyte as the dominant generation. It is monoicous: the small reddish sperm-producing antheridia are scattered along the midrib while the egg-producing archegonia grow nearer the tips of divisions of the plant.<ref>{{Harvnb|Watson|1981|pp=425–6}}</ref>
**** ''Antheridia and archegonia occur on different gametophytes,'' which are then called '''dioicous'''.<br />The moss ''Mnium hornum'' has the gametophyte as the dominant generation. It is dioicous: male plants produce only antheridia in terminal rosettes, female plants produce only archegonia in the form of stalked capsules.<ref>{{Harvnb|Watson|1981|pp=287–8}}</ref> Seed plant gametophytes are also dioicous. 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.{{sfn|Sporne|1974a|pp=17–21}}
* ''Differentiation of the spores.''
** ''All spores the same size'' ('''homospory''' or isospory).<br />Horsetails (species of ''[[Equisetum]]'') have spores which are all of the same size.<ref name="Bateman-1994-1">{{Harvnb|Bateman|Dimichele|1994|pp=350–1}}</ref>
** ''Spores of two distinct sizes'' ('''heterospory''' or anisospory): larger '''megaspores''' and smaller '''microspores'''. 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'''.<br />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 microgametophytes), and 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.e. sporophyte) has both kinds of flower and so is monoecious.[[Image:Hollyflowers.jpg|right|thumb|Flowers of European holly, a dioecious species: male above, female below (leaves cut to show flowers more clearly)]]
*** ''Megasporangia and microsporangia occur on different sporophytes,'' which are then called '''dioecious'''.<br />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. 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.<ref name="Bateman-1994-1"/>

=== A complex life cycle ===

[[File:Alternation of generations complex.svg|upright=2.5|thumb|Alternation of generations in a species which is heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious]]

Plant life cycles can be complex. Alternation of generations can take place in plants which are at once heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious, such as in a [[willow]] tree (as most species of the genus ''Salix'' are dioecious).<ref>{{cite EB1911|wstitle=Willow|volume=28|pages=688–689}}</ref> The processes involved are:
* 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.<br />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.<br />In a willow, microspores are not liberated from the anther (the microsporangium), but develop into pollen grains (microgametophytes) within it. The whole pollen grain is moved (e.g. by an insect or by the wind) to an ovule (megagametophyte), where a sperm is produced which moves down a pollen tube to reach the egg.
** 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.<br />Female willow trees (megasporophytes) produce flowers with only carpels (modified leaves that bear the megasporangia).
** Megaspores germinate producing megagametophytes; at maturity one or more archegonia are produced. Eggs develop within the archegonia.<br /> 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).{{-}}