<< Chapter < Page Chapter >> Page >

Alternation of generations

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

 The plant life cycle has haploid and diploid stages. The cycle begins when haploid (1n) spores undergo mitosis to form a multicellular gametophyte. The gametophyte produces gametes, two of which fuse to form a diploid zygote. The diploid (2n) zygote undergoes mitosis to form a multicellular sporophyte. Meiosis of cells in the sporophyte produces 1n spores, completing the cycle.
Alternation of generations between the haploid (1 n ) gametophyte and diploid (2 n ) sporophyte is shown. (credit: modification of work by Peter Coxhead)

Most plants exhibit alternation of generations, which is characterized by alternating between two generations: the gametophyte generation followed 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) whereas the root, shoot and leaf system that you see when looking at towering trees such as sequoias and pines is the sporophyte structure in more derived groups of plants.

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 protection and nutrients to the embryo as it develops into the new generation of sporophyte.

Sporophytes in the seedless plants

The sporophyte of seedless plants is diploid and results from the fusion of two gametes ( [link] ). 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 haploid spores are produced by meiosis, which reduces the 2 n chromosome number to 1 n . The spores are later released by the sporangia and disperse in the environment. After germinating from a spore, the gametophyte produces both male and female gametes, usually on the same individual.

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 ( [link] and [link] ).

 The fern life cycle begins with a diploid (2n) sporophyte, which is the fern plant. Sporangia are round bumps that occur on the bottom of the leaves. Sporangia undergo mitosis to form haploid (1n) spores. The spores germinate and grow into a green gametophyte that resembles lettuce. The gametophyte produces sperm and eggs that fuse to form a diploid (2n) zygote. The zygote undergoes mitosis to form a 2n sporophyte, ending the cycle.
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)
Sporogenous tissue undergoes meiosis to produce haploid (1n) spores, which germinate into young gametophytes. The gametophytes grow and develop into male or female gametophytes, which then produce sperm and eggs that fuse to form a diploid (2n) zygote. The zygote undergoes mitosis to form a 2n sporophyte, ending the cycle.
This life cycle of a moss shows alternation of generations with a dominant gametophyte stage. (credit: modification of work by Mariana Ruiz Villareal)

Gametophytes in the seedless plants

The gametophytes of seedless plants in which gametes are produced by mitosis. The male gametophyte: 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 gametophyte. The embryo develops inside the archegonium as the new sporophyte.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Which of the following is best at showing the life expandency of an individual within a a population
Daniel Reply
perianth is present in which gymnosperms ?
DebaXish Reply
perianth is present in which gymnos4perms ?
DebaXish Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Bi 101 for lbcc ilearn campus. OpenStax CNX. Nov 28, 2013 Download for free at http://legacy.cnx.org/content/col11593/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Bi 101 for lbcc ilearn campus' conversation and receive update notifications?

Ask