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Waarom is een fase van 'n plant dominant (afwisseling van generasie)?

Waarom is een fase van 'n plant dominant (afwisseling van generasie)?



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By plante is daar die afwisseling van generasie.

In byna alle landplante is een fase van die twee moontlike fases dominant - naamlik die sporofiet.

Die "oorheersing" oor die ander fase kan op die volgende maniere gesien word:

  • Die gametofiet organisme is afhanklik van sy sporofiet stamvader, terwyl die sporofiet organisme is goed sonder die ander.
  • Die sporofiete is meer struktureel kompleks, groter en leef langer.
  • Die sporofiete voer meer aktiwiteite/funksies uit en kry meer subjektiewe aandag.

Ek wil weet of daar 'n evolusionêre rede is waarom afwisseling van generasie, een fase dominant is. Hier is 'n paar raaiskote:

  1. Zipf se wet: namate een van die twee fases meer kompleks geword het (begin om meer komplekse werke te doen), het dit geriefliker geword en meer waarskynlik vir daardie fase om ander funksies te bekom, wat veroorsaak het dat die kompleksiteit van daardie fase toegeneem het met 'n soort positiewe-voer- terug-lus, terwyl dit meer voordelig geword het vir die ander fase om meer en meer van die ander afhanklik te wees, te verminder sy kompleksiteit.
  2. Chromosomale verskil: die sporofiete van plante is diploïed terwyl die gametofiete haploïed is, miskien is een van die twee konfigurasies meer doeltreffend?
  3. Die sporofietgenerasie "reproduseer" ongeslagtelik, miskien laat dit dit toe om 'n bietjie "voorsprong" te hê in die opbou van nie-seksueel voortplantingsfunksies en het meer kompleks geword soos in 1 verduidelik.

Kort antwoord:

Baie van die evolusionêre ontwikkelings in plante het in die sporofietlewestadium ontwikkel. Omdat verhoogde fiksheid van hierdie toenemend sporofiet-"dominante" plante 'n groter oorlewing en voortplantingsukses tot gevolg sou hê, het hierdie plante meer dominant geword as die meer beperkte gametofiet-dominante vroeë plante.

  • Met ander woorde, natuurlike seleksie van die sterkste plante (wat toevallig sporofieteienskappe het wat toenemend aangepas is om op land te lewe) het waarskynlik gelei tot die oorheersing in die sporofiet lewensfase.

Lang antwoord:

Ek sou vermoed dit is te wyte aan die evolusionêre voordele wat geassosieer word met die sporofietlewestadium in landplante. Veelvuldige voordele ontwikkel in die sporofietstadium van plante soos hulle ontwikkel het van eenvoudige gametofiet-dominante bryofiet-voorouers tot die groot verskeidenheid baie meer suksesvolle en dominante varings, gimnosperme en angiosperme.

Verskuiwing van gametofiet-gedomineerde lewensiklus (by briofiete) na sporofiet-gedomineerde lewensiklus. Bron: Cengage Learning (2016)

Evolusionêre ontwikkelings wat oor byna 500 miljoen jaar in landplante plaasgevind het, sluit die ontwikkeling van huidmondjies, ware blare, stuifmeel en sade, en blomme en vrugte in. Elkeen van hierdie ontwikkelings het plaasgevind in die sporofiet lewensfase.

Elkeen van hierdie evolusionêre eienskappe het 'n beduidende effek gehad op die sukses van die verskillende planttipes wat dit besit, soos afgelei kan word uit die toenemende sukses en dominansie van toenemend meer ontwikkelde plantafdelings.

Elkeen van hierdie sporofiet-gefokusde evolusionêre eienskappe het die voortplantingsukses van plante verhoog (dus swaaiend toenemende voordeel teenoor plante wat toenemend sporofiet-dominante lewensiklusse besit) as gevolg van hul vermoë om plantgrootte, groei en sukses te verhoog wat in droë (d.w.s. terrestriële) omgewings leef..

  • stomata maak voorsiening vir verhoogde gaswisseling met meer beperkte waterverlies (let wel: dit was ook nodig met die ontwikkeling van kutikula in sporofiete van vroeë briofiete)

  • vaskulêre weefsel2 laat plante toe om langer/groter/meer kompleks te groei terwyl hulle steeds water en voedingstowwe uit die grond ontvang

  • ware blare maak voorsiening vir groter energie-opname en groei (en dus groter mededingende voordele)

  • stuifmeel en sade maak voorsiening vir groter verspreiding en verwyder/beperk die beperkings van droogheid op bevrugting

  • blomme en vrugte verhoog bestuiwing en saadverspreiding

Die vermindering van die gametofietstadium tot klein selle binne groter sporofietplante laat hierdie andersins waternodige proses plaasvind in die droë toestande van lewe op land. As sodanig was die stuifmeeldraende plante (d.w.s. gimnosperme en angiosperme) baie suksesvol in die kolonisering van uitgestrekte terrestriële ekosisteme.

Nog 'n evolusionêre voordeel voorgestel, (sien bv. Bernstein et al. (1981)1 en Michod & Gayley (1992)3) is dat die diploïdie van die sporofietstadium laat maskering toe van die uitdrukking van skadelike mutasies deur genetiese aanvulling. Van Wikipedia:

As een van die ouerlike genome in die diploïede selle dus mutasies bevat wat lei tot defekte in een of meer geenprodukte, kan hierdie tekorte vergoed word deur die ander ouergenoom (wat nietemin sy eie defekte in ander gene kan hê). Soos die diploïede fase oorheersend geword het, het die maskeringseffek waarskynlik genoomgrootte, en dus inligting-inhoud, toegelaat om toe te neem sonder die beperking om die akkuraatheid van replikasie te verbeter. Die geleentheid om inligting-inhoud teen lae koste te verhoog, is voordelig omdat dit toelaat dat nuwe aanpassings geënkodeer word.

  • Hierdie voorstel is egter meer onlangs deur Szövényi *et al. (2013)4 omdat bewyse daarop dui dat seleksie nie meer effektief is in haploïede vs. diploïede lewensiklusfases van mosse en angiosperme nie.

Vir verdere lees:

  • "Evolusionêre geskiedenis van plante" [Wikipedia]

  • "Evolusie van saadplante" [Lumen Learning]

  • "Evolusie van saadplante" [OpenStaxCollege]


Aanhalings

1 Bernstein, H; Byers, GS; Michod, RE (1981). Evolusie van seksuele voortplanting: Belangrikheid van DNA-herstel, aanvulling en variasie. Die Amerikaanse natuurkundige. 117 (4):537-549. doi:10.1086/283734

2 Lucas, W.J., Groover, A., Lichtenberger, R., Furuta, K., Yadav, S.R., Helariutta, Y., He, X.Q., Fukuda, H., Kang, J., Brady, S.M. en Patrick, J.W., 2013. Die plant vaskulêre sisteem: evolusie, ontwikkeling en funksies F. Tydskrif vir integrerende plantbiologie, 55(4):294-388.

3 Michod, RE; Gayley, TW (1992). Maskering van mutasies en die evolusie van seks. Die Amerikaanse natuurkundige. 139 (4):706-734. doi:10.1086/285354

4 Szövényi, Péter; Ricca, Mariana; Hock, Zsófia; Shaw, Jonathan A.; Shimizu, Kentaro K. & Wagner, Andreas (2013). Seleksie is nie meer doeltreffend in haploïede as in diploïede lewensfases van 'n angiosperm en 'n mos nie. Molekulêre Biologie en Evolusie. 30 (8): 1929-39. doi:10.1093/molbev/mst095


Afwisseling van generasies

In diere, genereer meiose die haploïede gamete &mdash sperm en eiers &mdash direk. Hierdie enkelselle versmelt om die sigoot te vorm wat in 'n ander diploïede dier sal ontwikkel.

In die meeste plante meiose en bevrugting verdeel die lewe van die organisme in twee afsonderlike fases of "generasies".

  • Die gametofiet generasie begin met a spoor geproduseer deur meiose. Die spoor is haploïed, en al die selle wat daaruit afkomstig is (deur mitose) is ook haploïed. Mettertyd produseer hierdie meersellige struktuur gamete &mdash deur mitose &mdash en seksuele voortplanting produseer dan die diploïed sporofiet generasie.
  • Die sporofietgenerasie begin dus met 'n sigoot. Sy selle bevat die diploïede aantal chromosome. Uiteindelik sal sekere selle egter meiose ondergaan, spore vorm en 'n nuwe gametofietgenerasie begin.
  • Mitose kan voorkom in haploïede selle sowel as diploïede selle.
  • 'n Haploïede stel chromosome, en dus 'n enkele stel gene (een genoom), is voldoende om selfunksie in hierdie organismes te beheer (maar nie in die meeste diere nie).

Trouens, die gametofietgenerasie is die belangrikste stadium in die lewe van mosse en 'n onafhanklike plant in varings.

Skakels na beskrywings van die gametofiete van mosse en varings.

Die gametofiet is egter slegs 'n onopvallende struktuur in angiosperme en ander "hoër" plante.


Bryofiete: verspreiding, habitat en affiniteite | Plantkunde

In hierdie artikel sal ons bespreek oor: - 1. Betekenis van Bryofiete 2. Oorsprong van Bryofiete 3. Verspreiding 4. Habitat 5. Algemene karakters 6. Klassifikasie 7. Aanpassings 8. Amfibieë 9. Afwisseling van generasie 10. Risoïede en Dopluise 11. Archesporium 12. Affiniteite.

  1. Betekenis van Bryofiete
  2. Oorsprong van Bryofiete
  3. Verspreiding van Bryofiete
  4. Habitat van Bryofiete
  5. Algemene karakters van Bryofiete
  6. Klassifikasie van Bryofiete
  7. Aanpassings van Bryofiete na Landhabitat
  8. Bryofiete: Amfibieë van Planteryk
  9. Afwisseling van generasie in Bryofiete
  10. Risoïede en skubbe in Bryofiete
  11. Archesporium in Bryophytes
  12. Affiniteite van Bryofiete

1. Betekenis van Bryofiete:

Bryophyta (Gr. Bryon = massa phyton = plant), 'n afdeling van koninkryk Plantae wat bestaan ​​uit mosse, horingmosse en lewermosse. Hulle is groepe groen plante wat 'n posisie tussen die tallofiete (Alge) en die vaskulêre kriptogams (Pteridofiete) inneem.

Bryofiete produseer embrio's, maar het 'n gebrek aan sade en vaskulêre weefsels. Hulle is die mees eenvoudige en primitiewe groep van Embryophyta. Daar word gesê dat hulle die eerste landplante of nie-vaskulêre landplante (Atracheata) is. Die teenwoordigheid van swemmende anterozoïede is 'n bewys van hul akwatiese voorouers.

2. Oorsprong van Bryofiete:

Niks definitief is bekend oor die oorsprong van Bryofiete nie as gevolg van die baie min fossielrekord. Daar is twee sienings oor die oorsprong van Bryofiete.

(i) Alghipotese van die oorsprong van Bryofiete.

(ii) Pteridofietiese hipotese van die oorsprong van Bryofiete.

(i) Alghipotese van oorsprong:

Daar is geen fossielbewyse van oorsprong van Bryofiete van alge nie, maar Bryofiete lyk met alge in karakters soos amfibiese aard, teenwoordigheid van gevlageerde anterozoïede en die noodsaaklikheid van water vir bevrugting.

Hierdie hipotese is ondersteun deur Lignier (1903), Bower (1908), Fritsch (1945) en Smith (1955) ens. Volgens Fritsch (1945) en Smith (1955) is Bryofiete ontstaan ​​uit die heteroryke ​​groen alge wat tot die orde behoort. Chaetophorales vir bv. Fritschiella, Coleochaete en Draparnaldiopsis.

(ii) Pteridofitiese hipotese van oorsprong:

Volgens hierdie hipotese is Bryofiete afstammelinge van Pteridofiete. Hulle word uit Pteridofiete ontwikkel deur progressiewe vereenvoudiging of vermindering.

Hierdie hipotese is gebaseer op sekere karakters soos die teenwoordigheid van tipe huidmondjies op die sporogonium van Anthoceros en apofise van mosse soortgelyk aan die vaskulêre landplante, soortgelyk in die sporofiete van sommige Bryofiete (bv. Anthoceros, Sphagnum, Andreaea) met sommige lede van Psilophytales of Pteridophytes (bv. Rhynia, Hormophyton ens.)

Hierdie hipotese is ondersteun deur Scot (1911), Kashyap (1919), Kidston en Lang (1917-21), Haskell (1914) Christensen (1954), Proskaner (1961), Mehra (1968) ens.

3. Verspreiding van Bryofiete:

Bryofiete word verteenwoordig deur 960 genera en 24 000 spesies. Hulle is kosmopolities in verspreiding en groei beide in die gematigde en tropiese streke van die wêreld op 'n hoogte van 4000-8000 voet.

In Indië is Bryofiete redelik volop in beide Nilgiri-heuwels en Himalaja Kullu, Manali, Shimla, Darjeeling, Dalhousie en Garhwal is van die heuwelagtige streke wat ook 'n weelderige groei van Bryofiete het. Oos-Himalajas het die rykste in briofitiese flora. 'n Paar spesies Riccia, Marchantia en Funaria kom in die vlaktes van U.P., M.P. Rajasthan, Gujarat en Suid-Indië.

In heuwels groei hulle gedurende die somer of reënseisoen. Winter is die rusperiode. In die vlaktes is die rustyd somer, terwyl aktiewe groei gedurende die winter en die reënseisoen plaasvind. Sommige Bryofiete is ook aangeteken uit verskillende geologiese eras, bv. Muscites yallourensis (Coenozoic era), Intia vermicularies, Marchantia spp. (Paleosoïkum) ens.

4. Habitat van Bryofiete:

Bryofiete groei dig in klam en skaduryke plekke en vorm dik matte of matte op klam grond, klippe, bas van bome veral gedurende reënseisoen.

Die meerderheid van die spesies is terrestrisch, maar 'n paar spesies groei in vars water (akwaties), bv. Riccia fluitans, Ricciocarpos natans, Riella, ens. Bryofiete word nie in die see aangetref nie, maar sommige mosse groei in die skeure van rotse en word gereeld gebad. deur seewater, bv. Grimmia maritima.

Sommige Bryofiete groei ook in uiteenlopende habitats, bv. Sphagnum-groei in moerasse, Dendroceros-epiphytic, Radula protensa. Crossomitrium-epiphyllous, Polytrichum juniperinum-xerophytic, Tortula muralis-op ou mure. Tortula desertorum in woestyne, Porella platyphylla-op droë rotse, Buxbaumia aphylla (mos), Cryptothallus mirabilis (lewermos) is saprofieties.

5. Algemene karakters van Bryofiete:

1. Plantliggaam is gametofities, onafhanklik, dominant, outotrofies, óf talloïed (d.w.s. tallusagtig, nie gedifferensieer in wortel, stam en blare nie) óf blaar (Fig 1), wat 'n wortellose blaarloot bevat.

2. Plantliggaam is baie klein en wissel van 'n paar mm. tot baie cm. Zoopsis is die kleinste briofiet (5 mm.) terwyl die hoogste briofiet Dawsonia (50-70 cm.) is.

3. Blare en stingels wat in vaatplante gevind word, is afwesig, Koch (1956) noem dit ‘blaar’ en ‘stam’ soos strukture as ‘as’ en ‘phylloid’ onderskeidelik.

4. Wortels is afwesig. Funksies van die wortels word deur risoïede uitgevoer. Selle is ook in staat om vog direk uit die grond of atmosfeer te absorbeer. Daarom kan Bryofiete ook op die klam gronde oorleef.

5. Risoïede kan eensellig, onvertakte (bv. Riccia, Marchantia, Anthoceros) of meersellig en vertakt wees (bv. Sphagnum, Funaria).

6. In lede van orde Marchantiales (bv. Riccia, Marchantia) is toonlere teenwoordig. Dit is violetkleurig, meersellig en enkelseldik. Hulle beskerm die groeipunt en help om die vog te behou.

7. Vaskulêre weefsel (xileem en floëem) is heeltemal afwesig. Water en voedselmateriaal word van sel tot sel oorgedra. In sommige Bryofiete (bv. mosse) is 'n paar selle in groepe van 2-3 egter teenwoordig vir geleiding van water en voedsel (foto assimileer). Hierdie selle staan ​​bekend as hidroïed (gesamentlik hidrom) en leptoïede onderskeidelik. Kutikula en huidmondjies is afwesig.

6. Klassifikasie van Bryofiete:

Die term Bryophyta is die eerste keer deur Braun (1864) bekendgestel, maar hy het alge, swamme, ligene en mosse by hierdie groep ingesluit. Later is alge, swamme en ligene in 'n aparte afdeling Thallophyta en lewermosse, mosse in afdeling Bryophyta geplaas. Die rang van afdeling Bryophyta aan hierdie goed gedefinieerde groep plante is die eerste keer deur Schimper (1879) gegee.

Eichler (1883) was die eerste wat Bryophyta in twee groepe verdeel het:

Engler (1892) het Hepaticae en Musci as twee klasse erken en elke klas in die volgende drie ordes verdeel:

Afdeling. Bryophyta:

Klas I. Hepaticae verdeel in drie ordes:

Klas II. Musci verdeel in drie ordes:

As gevolg van geïsoleerde karakters van Anthoceros en verwante genera, het Howe (1899) die orde Anthocerotales tot die rang van 'n klas verhoog en die afdeling Bryophyta in drie klasse verdeel:

Hierdie stelsel van klassifikasie is gevolg deur Smith (1938, 1955), Takhtajan (1953), Wardlaw (1955) en Schutser (1958), maar het verkies om klas Anthocerotae as Anthocerotae te noem. Internasionale kode van Botaniese Nomenklatuur (ICBN) het in 1956 voorgestel dat die agtervoegsel-opsida vir die klasse gebruik moet word en sodanige gebruik is reeds deur Rothmaler (1951) vir die klasse van Bryofiete voorgestel.

Hy het die klasname verander as:

Klas I. Hapaticae as Hepaticopsida.

Klas II. Anthocerotae as Anthoceropsida

Klas III. Musci as Bryopsida.

Proskauer (1957) het voorgestel dat die klasnaam Antheoceropsida ch _ Anthocerotopsida moet wees. Parihar (1965) en Holmes (1986) het Proskauer se sysuaa-klassifikasie gevolg en Bryophyta in drie klasse verdeel: Klas I. Hepaticopsida Klas II. Anthocerotopsida Klas III. Bryopsida.

Klas I. Hepaticopsida (Lewerworts):

Algemene karakters:

1. Hierdie klas sluit ongeveer 280 genera en 9500 spesies in.

2. Die naam van hierdie klas is afgelei van 'n Latynse woord Hepatica wat lewer beteken. Lede van hierdie klas staan ​​dus algemeen bekend as lewerwortels.

3. Plantliggaam is gametofities en die gametofiet is óf thalloïed óf foliose.

4. Thalloïedvorms is prostreer, gelob, dorsiventraal en digotoom vertakt.

5. In blaarvorms, ‘blare’ is heel, gelob of verdeel en sonder ‘midrib’. ‘Blare gerangskik in twee tot drie rye op die as.

6. Risoïede is eensellig en vertakt.

7. Fotosintetiese selle bevat baie chloroplaste.

9. Geslagsorgane word dorsaal of apikaal gedra, oppervlakkig of ingebed in gametofietweefsel

10. Lede kan een- of tweehuisig wees.

11. Sporophyte is óf eenvoudig óf slegs deur kapsule voorgestel (bv. Riccia) of kan gedifferensieer word in voet, seta en kapsule (bv. Marchantia).

12. Archesporiuin is endotesiaal van oorsprong.

13. Sporogene weefsel vorm óf slegs spore (bv. Riccia) óf word gedifferensieer in steriele elater moederselle en vrugbare spoor moederselle.

14. Columella is afwesig in die kapsule.

15. Elaters is eensellig, higroskopies met spiraalverdikkings.

16. Kapsulewand is een tot verskeie lae dik en sonder huidmondjies.

17. Dehisensie van die kapsule is onreëlmatig of in 'n bepaalde aantal kleppe.

18. Spore op ontkieming vorm die gametofitiese plantliggaam.

19. Plante toon heteroinorfe afwisseling van generasie.

Campbell (1936) het die klas Hepaticopsida in vier ordes verdeel:

Orde 1. Marchantiales (bv. Riccia, Marchantia).

Orde 2. Sphaerocarpales (bv. Sphaerocarpos).

Orde 3. Jungermanniales (bv. Pellia).

Orde 4. Calobryales (bv. Calobryum).

Schuster (1953, 1958) het die klas Hepaticae in twee subklasse verdeel:

Subklas 1. Jungerinanniae. Dit sluit vier bestellings in:

Orde I. Calobryales (bv. Calobryum)

Orde 2. Takakiales (bv. Takakia)

Orde 3. Jungermanniales (bv. Pellia)

Orde 4. Metzgeriales (bv. Metzgeria)

Dit sluit drie bestellings in:

Orde 5. Sphaerocarpales (bv. Sphaerocarpos)

Orde 6. Monocleale (bv. Monoclea)

Orde 7. Marchantiales (bv. Marchantia).

Klas II. Anthocerotopsida (Hornworts):

Algemene karakters:

1. Hierdie klas word verteenwoordig deur ongeveer 6 genera en 300 spesies.

2. Plantliggaam is plat, dorsiventraal, thalloïed, gametofities en verskillend gelob.

3. Gladwandige risoïede is teenwoordig.

4. Tuberkulêre risoïede en skubbe is afwesig.

5. Intern word die tallus nie in sones gedifferensieer nie.

7. Lugkamers of lugporieë is afwesig.

8. Elke sel het 'n enkele chloroplast en elke chloroplast bevat 'n enkele pirenoïed.

9. Slymholtes maak oop op die ventrale oppervlak deur slymporieë.

10. Geslagsorgane is ingebed in die tallus.

11. Anteridia ontwikkel óf alleen óf in groepe in geslote holtes wat anteridiale kamers genoem word.

12. Die sporofiet word gedifferensieer in voet, 'n intermediêre sone of meristematiese sone en kapsule.

13. As gevolg van die teenwoordigheid van die meristematiese sone, toon die sporofiet onbepaalde groei d.w.s. dit bly onbepaald groei.

14. Archesporium is amfiteties van oorsprong.

15. Sporogene weefsel vorm die vrugbare spore en steriele elaters. Elaters het nie spiraalverdikkings nie en staan ​​bekend as pseudo-elaters.

16. Kapsulewand is vier tot ses lae dik en epidermis het die huidmondjies.

17. Kapsule verouder van top tot basis en skei gewoonlik deur twee kleppe.

Die klas Anthocerotopsida het slegs 'n enkele orde Anthocerotales. Muller (1940), Proskauer en Reimers (1954) het die orde Anthocerotales in twee families verdeel:

Familie 1. Anthocerotaceae (bv. Anthoceros)

Familie 2. Notothylaceae (bv. Notothylas).

Klas III. Bryopsida (Mosses):

Algemene karakters:

1. Dit is die grootste klas in Bryophyta en sluit ongeveer 700 genera en 14 000 spesies in.

2. Die hoofplantliggaam is gametofities en kan in twee stadiums gedifferensieer word - jeugdige stadium en blaarstadium of gametofoor.

3. Jeugstadium word voorgestel deur groen, filamentagtige vertakte strukture wat protonema genoem word. Dit ontwikkel uit die ontkieming van die spoor.

4. Gametofore is regop blaartakke wat op die protonema ontwikkel.

5. Gametofore kan vertakt of onvertakt wees en kan gedifferensieer word in drie dele - risoïede, ‘stam’ en ‘blare’.

6. Takke ontstaan ​​onder die ‘blare’.

7. ‘Blae’ is met middelrif, ongelob en spiraalvormig in drie tot agt rye op die as gerangskik of

8. Risoïede is meersellig, filamentagtig, vertakt met skuins septa.

9. Die as word gedifferensieer in sentrale geleidende string wat deur korteks omring word.

10. Geslagsorgane apikaal gedra in die groepe op hoof ‘stam’ of 'n tak.

11. Die sporofiet is groen in vroeë stadiums en kan gedifferensieer word in voet, seta en kapsule.

12. Die seta is gewoonlik verleng en styf.

13. Columella is gewoonlik teenwoordig en endotesies van oorsprong.

14. Archesporiurn (spoorvormende weefsel) word slegs in spore gedifferensieer.

16. Ontbinding van kapsule vind plaas deur skeiding van deksel of operculum.

17. Peristoom help met die verspreiding van spore.

18. Spore op ontkieming produseer die protonema.

Bower (1935), Wettstein (1933-1935), Campbell (1940) het die klas Bryopsida verdeel in die ordes:

Dixon (1932) het bogenoemde bevele die rang van subklas gegee en die Bryopsida in drie subklasse verdeel:

Smith (1938, 1955) het die klas Bryopsida in drie subklasse verdeel:

Reimers (1954) het die klas Bryopsida in 5 sub-klasse verdeel en hy het agtervoegsel-idae vir die sub-klas gebruik:

Sub-klas 1. Sphagnidae-1 orde. Sphagnales-1 familie.

Subklas 2. Andreaeidae-1 orde. Andreaeales-1 gesin.

Sub-klas 3. Bryidae-12 bestellings

Subklas 4. Buxbaumiidae-1 orde. Buxbaumilaes-2 gesinne.

Subklas 5. Polytrichidae-2-ordes. Polytrichales en Dawsionales-2 families.

Parihar (1955) het die klas Bryopsida in 3 subklasse verdeel:

Verskeie karakters word gebruik vir die klassifikasie van Bryofiete.

Sommige belangrike karakters is:

(i) Eksterne en interne struktuur van die tallus.

(iv) Posisie van geslagsorgane.

(v) Struktuur en aard van sporofiet.

(vi) Graad van sterilisasie in die sporofiet.

7. Aanpassings van Bryofiete by Landgewoonte:

Bryofiete is eerste landplante. Bewyse ondersteun dat Bryofiete uit alge ontwikkel is. Tydens die proses van oorsprong het hulle ontwikkel tot sekere aanpassings aan landgewoontes.

1. Ontwikkeling van kompakte plantliggaam bedek met epidermis.

2. Ontwikkeling van organe vir aanhegting en absorpsie van water, bv. risoïede.

3. Absorpsie van koolstofdioksied uit atmosfeer vir fotosintese, bv. lugpore.

4. Beskerming van voortplantingselle teen uitdroging en meganiese besering, dit wil sê geslagsorgane met baadjie.

5. Retensie van sigoot binne die argegonium.

6. Produksie van groot aantal dikwandige spore.

7. Verspreiding van spore deur wind.

8. Bryofiete: Amfibieë van Planteryk:

Bryofiete staan ​​ook bekend as amfibieë van die planteryk omdat water nodig is om die lewensiklus te voltooi. In diereryk klas Amfibieë (Gr. Amphi = twee of albei bios = lewe) sluit daardie gewerwelde diere in wat amfibieë in die natuur is, dit wil sê hulle kan op land sowel as in water leef. Net so is die meerderheid van die briofiete terrestreel maar hulle is onvolledig aangepas by die landtoestande.

Hulle kan nie gedurende droë seisoen groei nie en benodig voldoende hoeveelheid water vir hul vegetatiewe groei. Water is absoluut noodsaaklik vir die volwassenheid van geslagsorgane ar. bevrugting. Sonder water is hulle nie in staat om hul lewensiklus te voltooi nie. Vanweë hul komplekse afhanklikheid van eksterne water vir die voltooiing van hul lewensiklus, word Bryofiete saam met Pteridofiete as amfibieë van die planteryk beskou.

9. Afwisseling van generasie in Bryofiete:

Bryofiete toon 'n duidelike en skerp gedefinieerde heteromorfiese afwisseling van generasie. In die lewensiklus van hierdie plante bestaan ​​daar twee afsonderlike fases. Een is haploïede (X) of gametofitiese fase (produseer gamete). Dit is die dominante en onafhanklike fase van die lewensiklus. Dit produseer die manlike en vroulike geslagsorgane, dit wil sê antheridia en archegonia onderskeidelik.

Haploïede gamete d.w.s. anterozoïede en eiers word binne die geslagsorgane geproduseer. Anterozoïede word in antheridia geproduseer en eiers word in archegonia geproduseer. Die gamete versmelt om 'n diploïede (2x) sigoot te vorm. Die sigoot is die beginpunt van die volgende fase van die lewensiklus.

By ontkieming vorm die sigoot die tweede diploïede volwassene van die lewensiklus wat sporofiet of sporogonium genoem word. Sporogonium produseer spoormoederselle in die kapsulestreek, wat meiose ondergaan en die haploïede spore vorm wat meiospore genoem word. Die sigoot, embrio, sporogonium en spoormoederselle vorm saam die sporofitiese generasie.

Hierdie generasie is heeltemal of gedeeltelik afhanklik van die gametofitiese generasie vir sy voeding. Elke meiospoor ontkiem en produseer 'n gametofietplant wat weer die geslagsorgane dra. Op hierdie manier gaan die lewensiklus voort. Omdat die twee generasies (gametofities en sporofities) afwisselend in die lewensiklusse voorkom, toon Bryofiete afwisseling van generasie.

Aangesien die generasies heeltemal verskil in hul morfologie, dit wil sê gametofiet is óf thalloïed óf foliose, en die sporofiet bestaan ​​gewoonlik uit voet, seta en kapsule, word dit heteromorfiese afwisseling van generasie genoem.

Apogamie en Aposporie in Bryofiete:

Bryofiete is toegerus met 'n merkwaardige regenerasievermoë. Dele van die plant of enige lewende sel van die tallus is in staat om die hele plant te regenereer. Die sporofitiese selle regenereer om 'n protonema te vorm waarop gametofiete verskyn. Hierdie regenerasie van diploïede gametofiet vanaf 'n sporofiet sonder die vorming van spore word aposporie genoem.

Omgekeerd kan 'n gametofiet 'n massa selle vorm wat 'n sporofiet kan regenereer. Hierdie regenerasie van 'n diploïede sporofiet uit 'n gametofiet, sonder die vorming van gamete, word apogamie genoem. Aposium en apogamie word selde in die lewensiklus van Bryofiete aangetref.

10. Risoïede en skubbe in Bryofiete:

Risoïede:

By Bryofiete is wortels afwesig en die funksies van die wortel d.w.s. verankering en absorpsie word uitgevoer deur die filamentagtige strukture bekend as risoïede.

Risoïede kan eensellig, onvertakte (Fig. 3B-D) in thallose-vorme van Hepaticopsida en Anthocerotopsida (bv. Riccia, Marchantia, Anthoceros) of meersellig wees en vertakt in foliose vorms van Bryopsida (Fig. 3 E) (bv. Funaria) , Polytrichum) Meersellige wortelstokke besit skuins dwarswande.

Eensellige risoïede is van twee tipes gladwandig en tuberkulêr (Fig. 3 B-D). Die lede van die orde Marchantiales (bv. Riccia, Marchantia) besit beide tipes risoïede terwyl Anthocerotales (bv. Anthoceros) slegs gladwandige wortelstokke besit.

In thalloïede vorms word risoïede op die ventrale oppervlak (Fig. 3 A) langs die middelrib gedra, maar in foliose vorms ontstaan ​​risoïede vanaf die basis van die ‘stam’. By akwatiese Bryofiete (bv. Riccia fluitans, Ricciocarpus natans) is risoïede afwesig.

Skubbe:

Skubbe is slegs teenwoordig in die lede van orde Marchantiales en afwesig in alle Bryofiete. Die skubbe is meersellig, violetkleurig en enkelseldik. Hulle is violet van kleur as gevolg van die teenwoordigheid van die pigment antosianien. Skubbe ontwikkel op die ventrale oppervlak van die tallus (Fig 3A).

Hulle kan in een ry gerangskik word (bv. jong tallus van Riccia) of in twee rye aan elke kant van die middelrib (bv. Targionia) of in twee tot vier rye aan elke kant van die middelrib (bv. Marchantia) of onreëlmatig versprei oor die hele ventrale oppervlak (bv. Corsinia).

In Riccia is die skubbe ligulaat (Fig. 3G) terwyl in Marchantia die skubbe van twee tipes is-ligulaat en appendikuleer (gedeel deur 'n nou vernouing in twee dele—liggaam en aanhangsel, Fig. 3F). Dopluise beskerm die groeipunt deur hul delikate selle te bedek en slym af te skei om hulle klam te hou. Die skubbe is afwesig in sommige akwatiese lede van orde Marchantiales, bv. Riccia fluitans.

11. Archesporium in Bryophytes:

Die archesporium is die eerste selgenerasie van die sporogene weefsel. Dit verdeel en herverdeel om 'n massa selle te vorm. Dit is 'n soliede weefsel en word ook sporogene weefsel genoem. Die selle van die laaste selgenerasie van soliede weefsel skei van mekaar en staan ​​bekend as spoormoederselle. Die oorsprong, posisie en lot van archesporium verskil in verskillende lede van Bryophytes.

12. Affiniteite van Bryofiete:

Vanuit evolusionêre oogpunt beklee Bryofiete 'n tussenposisie tussen die Alge en die Pteridofiete. Hulle toon affiniteite met beide alge en pteridofiete.

Ooreenkomste van Bryofiete met alge:

1. Plantliggaam eenvoudig, talloïed en gametofities.

3. Gametofitiese fase is dominant.

5. Selwand bestaan ​​uit sellulose.

6. Pigmente (chlorofil a, chlorofil b, α en β karoteen, Lutin, Violaxanthes en Xeoxanthin) is soortgelyk in chloroplast.

7. Vaskulêre weefsel is afwesig.

8. Anterozoïede is beweeglik (twee-gevlag).

9. Flagella is whiplash tipe.

10. Water is noodsaaklik vir bemesting.

11. 'n Filamentagtige protonema word geproduseer deur Bryofiete (jeugstadium in mosse) wat ooreenstem met die filamentagtige groen alge.

12. In volgorde van Anthocerotales van Bryophytes, is plastiede met pirenoïede wat 'n kenmerk van Chlorophyceae (Groenalge) is.

Ooreenkomste van Bryofiete met Pteridofiete:

2. Primitiewe eenvoudige blaarlose en wortellose sporofiete van Pteridophytes (lede van orde Psilophytales) kan vergelyk word met die sporofiete van Bryophytes.

3. Seksuele voortplanting is oogamous.

4. Androsiete word omring deur steriele baadjielaag.

5. Anterozoïede is gevlag.

6. Water is noodsaaklik vir bemesting.

7. Permanente retensie van sigoot binne die argegonium.

8. Sigoot vorm die embrio.

9. Moskapsule is soortgelyk aan terminale sporangium en columella van Psilophytales.

10. Beide Bryofiete en Pteridofiete word gekenmerk deur heteromorfiese afwisseling, van generasie.


Waarom word dit geslagswisseling genoem?

Klik om verder te verken. Ook gevra, wat beteken afwisseling van generasie?

Afwisseling van geslagte (ook bekend as metagenese) is die tipe lewensiklus wat voorkom in daardie plante en alge in die Archaeplastida en die Heterokontophyta wat afsonderlike haploïede geslagtelike en diploïede ongeslagtelike stadiums het. Die haploïede spore ontkiem en groei tot 'n haploïede gametofiet.

Tweedens, wat is 'n voorbeeld van afwisseling van geslagte? Die varing is 'n voorbeeld van afwisseling van geslagte, waarin beide 'n meersellige diploïede organisme en 'n meersellige haploïede organisme voorkom en aanleiding gee tot die ander. Omdat die haploïede organisme gamete skep, word dit die gametofiet genoem generasie van die lewensiklus.

Eenvoudig so, het mense afwisseling van geslagte?

Mense doen nie het an afwisseling van geslagte omdat daar geen meersellige haploïede stadium is nie. Ek weet van slegs 'n baie min dierspesies met 'n meersellige haploïede stadium in die lewensiklus, en in daardie gevalle is die haploïede stadium steriel. Sulke organismes uitstal die verskynsel bekend as afwisseling van geslagte." bl.

Wat is die voordeel van afwisseling van geslagte?

Teoreties voordele van afwisseling van generasies Een voordeel hiervan is dat 'n mutasie wat 'n dodelike, of skadelike, eienskapuitdrukking veroorsaak, die gametofiet sal laat sterf, dus kan die eienskap nie na die toekoms oorgedra word nie geslagte, die behoud van die sterkte van die genepoel.


Waarom is een fase van 'n plant dominant (afwisseling van generasie)? - Biologie

Die term Bryophyta is die eerste keer in 1864 deur Robert Brawn geskep. Bryofiete is die eenvoudige en primitiewe groep van die klam landplant. Hierdie lede word algemeen amfibiese plante genoem omdat water noodsaaklik is vir bevrugting. Hierdie groep word tussen alge en Pteridophyta geplaas. Lid van bryofiet verteenwoordig ongeveer 25 000 spesies. 653 spesies word ook in Nepal aangetref. Bryophyta includes three different categories of plants i.e. liverworts, hornworts, and mosses.

General characteristics of Bryophyta

  • Bryophytes are commonly found in moist, shady, damp and cool places such as moist rocks, moist walls, moist soil surface and wooden logs.
  • In the primitive group of bryophytes, the vegetative plant body is green and leafy thallus is present. Type of thallus is prostrate, dorsoventrally flattened and dichotomously branched. But in the advanced member of bryophyte vegetative plant body is aerial, erect and differentiated into rhizoids, axis, and leaves.
  • True rot is absent but for anchorage and absorption they bear unicellular or multicellular rhizoids.
  • Conducting tissue or xylem and phloem are absent.
  • They reproduce by vegetative and sexual methods. Vegetative multiplication is by fragmentation, tuber formation, adventitious branches, and gemma cup. Sexual reproduction is by monogamous type. The male reproductive organ is antheridium and female reproductive organ is archegonium.
  • Male gametes are biflagellate and motile.
  • Water is essential for fertilization.
  • Reproductive organs are multicellular and jacketed.
  • Fusion product of male and female gametes is a diploid zygote. It develops sporophyte.
  • They show distinct alternation of generation and embryo stage is present.

Alternation of generation in Bryophyta

In the life cycle of bryophytes, gametophyte and sporophyte generation are regularly alternate with each other to complete life cycle. Such a phenomenon is called alternation of generation. Gametophyte phase is dominant, independent, autotrop[hic and haploid. It reproduces by male and female gamete formation. Fusion product of male and female gamete is a diploid zygote. A zygote is the mother cell of sporophyte generation. It develops sporophyte. Sporophyte phase is reduced, diploid and depend on the gametophyte. It reproduces by haploid spore formation. Germination of haploid spore again gives rise to young gametophyte.

Classification of Bryophyta

Bryophyta is divided into three classes on the basis of gametophytic, thallus and sporophytic generation. Hulle is:

Member of Anthocerotopsida is commonly called hornworts so its sporophytic phase is elongated, cylindrical and horn-like.

Rhizoids are multicellular and branched.

Reproductive organ is embed in the dorsal surface of the thallus.

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Dinge om te onthou
  • Bryophytes are commonly found in moist, shady, damp and cool places such as moist rocks, moist walls, moist soil surface and wooden logs.
  • Conducting tissue or xylem and phloem are absent.
  • Water is essential for fertilization.
  • Reproductive organs are multicellular and jacketed.
  • Fusion product of male and female gametes is a diploid zygote. It develops sporophyte.
  • They show distinct alternation of generation and embryo stage is present.
  • Bryophyta is divided into three classes on the basis of gametophytic, thallus and sporophytic generation.
  • It includes every relationship which established among the people.
  • Daar kan meer as een gemeenskap in 'n samelewing wees. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • gemeenskaplike belange en gemeenskaplike doelwitte is nie nodig vir die samelewing nie.

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Can You Explain The Significance Of Alternation Of Generation?

Alternation of generation is very significant because:It promotes the chances of survival of organisms.The populations become increasingly better adapted to environment.

Better chance for survival:

During the formation of spores from spore mother cells by meiotic division reshuffling of genes occurs. As consequences, a great variety of spores with different genetic make up are produced. These spores in turn produce gametophytes with different genetic combinations. The gametophytes with better genetic make up will have a better chance for survival in the environment. On the other hand, the gametophytes with less advantages characteristics will be eliminated. There is no reshuffling of genes during gametogensis in gametophyte as gametes are produced after mitosis.

Better adapted to environment:

The oospore developing after fertilization now has a new genetic make up as compared to the parent. This genetic variation passes to the new sporophytes which on maturity once again produces further genetic recombination which is transferred to the gametophytes. In the long run, this will allow the populations to become increasingly better adapted to their environment.

It is the phenomenon in the life cycle of many plants in which haploid gametophytes and diploid sporophyte regularly alternates with each other.

Verduideliking:
In the life history of liverworts, mosses and hornworts there are two distinct phases or generations.

Gametophyte:
The gametophytes is the dominate generation because it is more conspicuous. It produces gametes called spermatozoids and eggs, therefore called gametes producing generation. A haploid spermatozoid fuses with a haploid egg to produce diploid oospore.The oospore produces a totally different plant called sporophyte (also called sporogonium).

Sporophytes:
The sporophyte is a less conspicuous generation, which is usually differentiated into foot, seta and capsule. Spores develop with in the capsule by meiosis from spore mother cells. The sporophyte produces spores and is, therefore, called spore producing generation. Each spore on germination gives rise to the gametophyte.

Thus in the life history of a bryophytic plant, the two generations, the gametophyte and the sporophyte, regularly alternate with each other. The phenomenon of alternation of gametophte and sporophyte in the life history of plant is called alternation of generations.

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Content: Vascular Vs Nonvascular Plants

Vergelykingskaart

EienskappeVaatplanteNonvascular Plants
DefinisieVascular plants are the green plants of varying shapes and sizes, which comprise a specialized xylem and phloem vessels for water, minerals and food conduction, and along with that also possess a true root and shoot systemNon-vascular plants are the green and microscopic plants with poorly developed root and shoot system, and do not have a vascular system or the mechanics for water and food transportation
GrootteThese plants grow larger in size This group of plants are generally microscopic or grow relatively smaller compared with non-vascular plants
AbsorpsieThese possess deep roots that are specialized to absorb water through osmosisThese plants lack deep root system and solely depend on osmosis and diffusion to absorb water passively
Root SystemVascular plants possess a true root system that supports the plant body by absorbing water and essential minerals from the soil that are needed for the plant growth and developmentNonvascular plants possess shallow roots or rhizoids instead of a true root system
Dominant generation phaseIts principal generation phase is sporophyte that lasts longerIn nonvascular plants, the gametophyte phase is the dominant phase that persists longer
Stem/Shoot systemIt contains a well-developed shoot systemIt is devoid of true shoot system, as it only possess small leaves and lacks true stem, root, flowers, fruits, wood etc.
ReproduksieIt is achieved via seedIt is achieved via spores
BlareVascular plants possess true leaves that have cuticle, epidermis, meristematic cells, and stomata that are specialized to do distinct functionsIt lacks true leaves and specialized cells or tissues
Growth habitatThese can grow in variety of habitatsThese can grow in swampy, marshy, and damp places
VoorbeeldeIt includes clubmosses, grasses, sunflower, pines, horsetails, true ferns, angiosperms and gymnospermsIt includes mosses, green algae, liverworts and hornworts

Definition of Vascular Plants

Vascular plants or tracheophytes constitute a large group of terrestrial plants that carry specialized vaartuie (xylem and phloem), which are well distributed in the roots, stems, and leaves. Xylem and phloem cells constitute the vaskulêre stelsel and aid the translocation of food and water all over the plant body. Besides, the vascular system also provides support and rigiditeit to the plant.

Vascular plants possess well-developed vascular tissues, meristematic tissues, ground tissues, and dermal tissues. The life cycle of a vascular plant has alternations of two generations, in which a diploid sporophyte phase lasts longer. There is a characteristic feature of vascular plants that comprise a true root system and shoot system. Vascular plants include ferns, conifers, and flowering plants. The plants belong to this group have diverse and complex life cycles.

The vascular vessels in vascular plants are of two kinds, depending on what they transport. Die phloem vessels transport the photosynthetic food material to the rest of the plant body. In kontras, xylem vascular vessel aids the conduction of water from roots up to the whole plant. It includes trees, shrubs, grasses, flowering plants, and ferns.

Definition of Nonvascular Plants

Nonvascular plants or bryophytes form a group of aquatic and terrestrial plants that are without any specialized vessels (phloem and Xylem) for water and mineral conduction. It include green-algae, mosses, ferns, liverworts, and hornworts. They are considered as the lower plants, as these have neither true leaves, stem, root, flowers, and fruits nor specialized tissues for water and food conduction.

For the water translocation, nonvascular plants have simple tissues. A haploid gametophyte generation is prominent in the life cycle of nonvascular plants. These appear microscopic and grow very small. Instead of roots, it contains risoïede, which only support the plant body and perform no special role in water absorption.

Nonvascular plants lack deep roots that absorb water, so to combat this water requirement these are generally found in the moist environments so that they remain in touch with the water source. The reproductive strategy of nonvascular plants is quite different, as these can reproduce sexually via single-celled spore or asexually by ontluikende en fragmentasie.


Plant Adaptations to Life on Land

As organisms adapt to life on land, they have to contend with several challenges in the terrestrial environment. Water has been described as “the stuff of life.” The cell’s interior—the medium in which most small molecules dissolve and diffuse, and in which the majority of the chemical reactions of metabolism take place—is a watery soup. Desiccation, or drying out, is a constant danger for an organism exposed to air. Even when parts of a plant are close to a source of water, their aerial structures are likely to dry out. Water provides buoyancy to organisms that live in aquatic habitats. On land, plants need to develop structural support in air—a medium that does not give the same lift. Additionally, the male gametes must reach the female gametes using new strategies because swimming is no longer possible. Finally, both gametes and zygotes must be protected from drying out. The successful land plants evolved strategies to deal with all of these challenges, although not all adaptations appeared at once. Some species did not move far from an aquatic environment, whereas others left the water and went on to conquer the driest environments on Earth.

To balance these survival challenges, life on land offers several advantages. First, sunlight is abundant. On land, the spectral quality of light absorbed by the photosynthetic pigment, chlorophyll, is not filtered out by water or competing photosynthetic species in the water column above. Second, carbon dioxide is more readily available because its concentration is higher in air than in water. Additionally, land plants evolved before land animals therefore, until dry land was colonized by animals, no predators threatened the well-being of plants. This situation changed as animals emerged from the water and found abundant sources of nutrients in the established flora. In turn, plants evolved strategies to deter predation: from spines and thorns to toxic chemicals.

The early land plants, like the early land animals, did not live far from an abundant source of water and developed survival strategies to combat dryness. One of these strategies is drought tolerance. Mosses, for example, can dry out to a brown and brittle mat, but as soon as rain makes water available, mosses will soak it up and regain their healthy, green appearance. Another strategy is to colonize environments with high humidity where droughts are uncommon. Ferns, an early lineage of plants, thrive in damp and cool places, such as the understory of temperate forests. Later, plants moved away from aquatic environments using resistance to desiccation, rather than tolerance. These plants, like the cactus, minimize water loss to such an extent they can survive in the driest environments on Earth.

In addition to adaptations specific to life on land, land plants exhibit adaptations that were responsible for their diversity and predominance in terrestrial ecosystems. Four major adaptations are found in many terrestrial plants: the alternation of generations, a sporangium in which spores are formed, a gametangium that produces haploid cells, and in vascular plants, apical meristem tissue in roots and shoots.

Alternation of Generations

Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages (Figure 1).

Figuur 1: Alternation of generations between the haploid (1n) gametophyte and diploid (2n) sporophyte is shown. (credit: modification of work by Peter Coxhead)

Haplontic refers to a life cycle in which there is a dominant haploid stage. Diplontic refers to a life cycle in which the diploid stage is the dominant stage, and the haploid chromosome number is only seen for a brief time in the life cycle during sexual reproduction. Humans are diplontic, for example. Most plants exhibit alternation of generations, which is described as haplodiplontic : the haploid multicellular form known as a gametophyte is followed in the development sequence by a multicellular diploid organism, the sporophyte . The gametophyte gives rise to the gametes, or reproductive cells, by mitosis. It can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain in the higher plants (the collective term for the vascular plants). The sporophyte stage is barely noticeable in lower plants (the collective term for the plant groups of mosses, liverworts, and hornworts). Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines.

Sporangia in the Seedless Plants

The sporophyte of seedless plants is diploid and results from syngamy or the fusion of two gametes (Figure 1). The sporophyte bears the sporangia (singular, sporangium), organs that first appeared in the land plants. The term “sporangia” literally means “spore in a vessel,” as it is a reproductive sac that contains spores. Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, which reduces the 2n chromosome number to 1n. The spores are later released by the sporangia and disperse in the environment. Two different types of spores are produced in land plants, resulting in the separation of sexes at different points in the life cycle. Seedless nonvascular plants (more appropriately referred to as “seedless nonvascular plants with a dominant gametophyte phase”) produce only one kind of spore, and are called homosporous . After germinating from a spore, the gametophyte produces both male and female gametangia , usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores. The male spores are called microspores because of their smaller size the comparatively larger megaspores will develop into the female gametophyte. Heterospory is observed in a few seedless vascular plants and in all seed plants.

When the haploid spore germinates, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte or vegetative form, and the cycle begins anew (Figure 2 and Figure 3).

Figuur 2: This life cycle of a fern shows alternation of generations with a dominant sporophyte stage. (credit “fern”: modification of work by Cory Zanker credit “gametophyte”: modification of work by “Vlmastra”/Wikimedia Commons) Figuur 3: This life cycle of a moss shows alternation of generations with a dominant gametophyte stage. (krediet: wysiging van werk deur Mariana Ruiz Villareal)

The spores of seedless plants and the pollen of seed plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin. This substance is characterized by long chains of organic molecules related to fatty acids and carotenoids, and gives most pollen its yellow color. Sporopollenin is unusually resistant to chemical and biological degradation. Its toughness explains the existence of well-preserved fossils of pollen. Sporopollenin was once thought to be an innovation of land plants however, the green algae Coleochaetes is now known to form spores that contain sporopollenin.

Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered from desiccation and other environmental hazards. In both seedless and seed plants, the female gametophyte provides nutrition, and in seed plants, the embryo is also protected as it develops into the new generation of sporophyte.

Gametangia in the Seedless Plants

Gametangia (singular, gametangium) are structures on the gametophytes of seedless plants in which gametes are produced by mitosis. The male gametangium, the antheridium, releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia, the female gametangium. The embryo develops inside the archegonium as the sporophyte.

Apical Meristems

The shoots and roots of plants increase in length through rapid cell division within a tissue called the apical meristem (Figure 4). The apical meristem is a cap of cells at the shoot tip or root tip made of undifferentiated cells that continue to proliferate throughout the life of the plant. Meristematic cells give rise to all the specialized tissues of the plant. Elongation of the shoots and roots allows a plant to access additional space and resources: light in the case of the shoot, and water and minerals in the case of roots. A separate meristem, called the lateral meristem, produces cells that increase the diameter of stems and tree trunks. Apical meristems are an adaptation to allow vascular plants to grow in directions essential to their survival: upward to greater availability of sunlight, and downward into the soil to obtain water and essential minerals.

Figuur 4: This apple seedling is an example of a plant in which the apical meristem gives rise to new shoots and root growth.


Alternation of generation in archegoniates

Altrenation of generations:
All plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic (asexual) division. The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and give rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations.

Bryophyte generations
Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival (see Plant Classification, Bryophytes . Like all plants, the bryophyte life cycle goes through both haploid (gametophyte) and diploid (sporophyte) stages. The gametophyte comprises the main plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is attached to the gametophyte. The haploid stage, in which a multicellular haploid gametophyte develops from a spore and produces haploid gametes, is the dominant stage in the bryophyte life cycle. The mature gametophyte produces both male and female gametes, which join to form a diploid zygote. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues.
Tracheophyte Generations
Tracheophytes are plants that contain vascular tissue two of the major classes of tracheophytes are gymnosperms (conifers) and angiosperms (flowering plants). Tracheophytes, unlike bryophytes, have developed seeds that encase and protect their embryos. The dominant phase in the tracheophyte life cycle is the diploid (sporophyte) stage. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte (cones in gymnosperms and flowers in angiosperms), produce two different kinds of haploid spores: microspores (male) and megaspores (female). This phenomenon of sexually differentiated spores is called heterospory. These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes. Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte.


Alternation of Generations

For sexually reproducing multicellular organisms such as plants and animals, the life cycle requires that diploïed cells divide by meiose te skep haploïed selle. Haploid cells then fuse to recreate the diploid number and a new organism. Alternation of generations refers to the occurrence in the plant life cycle of both a multicellular diploid organism and a multicellular haploid organism, each giving rise to the other. This is in contrast to animals, in which the only multicellular phase is the diploid organism (such as the human man or woman), whereas the haploid phase is a single egg or sperm cell.

Alternation of generations is easiest to understand by considering the fern. The large, leafy fern is the diploid organism. On the undersurface of its fronds or leaves, its cells undergo meiosis to create haploid cells. However, these cells do not immediately unite with others to recreate the diploid state. Instead, they are shed as spores and germinate into small haploid organisms. Because the diploid organism creates spores, it is called the sporophyte generation of the life cycle. Upon reaching maturity, the haploid organism creates haploid egg and sperm cells (gametes) by mitose . Because the haploid organism creates gamete , it is called the gametophyte generation of the life cycle. The male gametes (sperm) are then released and swim to the female egg. Fusion of the gametes creates the new diploid sporophyte, completing the life cycle.

Whereas the fern gametophyte and sporophyte generations are completely independent, in some types of plants one generation lives on or in the other and depends on it for nutrition. In mosses, the familiar lush carpet of moss is the gametophyte, and its gametes require a moist environment for short-distance swimming before fusing. The sporophyte lives as a thin stalk attached to the gametophyte. Spores are released into the air and can travel on the slightest breezes to other habitats.

In contrast, in flowering plants (angiosperms), the sporophyte is the dominant form. The male gametophyte has been reduced to just three cells, two of which are sperm. These together form the pollen grain, which is formed from the anther, part of the sporophyte. Similarly, the female gametophyte has been reduced to just seven cells, one of which is the egg cell. These are retained inside the ovule , which is part of the sporophyte. In angiosperms, two bevrugting events take place: one sperm fertilizes the egg to form the diploid sigoot of the new individual, and the other sperm fertilizes the so-called polar nuclei to form the triploid endosperm , a nutritive tissue. Together with maternal sporophyte tissue, these make up the seed.


Kyk die video: Monique Bekker. Tutorial 1: Drie fasen voor groepsontwikkeling (Augustus 2022).