PLANT REPRODUCTION

Plant reproduction is the production of new individuals or offspring in plants,

which can be accomplished by sexual or asexual means. Sexual reproduction produces
offspring by the fusion of gametes, resulting in offspring genetically different from the
parent or parents. Asexual reproduction produces new individuals without the fusion of
gametes, genetically identical to the parent plants and each other, except when
mutations occur. In seed plants, the offspring can be packaged in a protective seed,
which is used as an agent of dispersal.
Plants have two main types of asexual reproduction in which new plants are
produced that are genetically identical clones of the parent individual. "Vegetative"
reproduction involves a vegetative piece of the original plant (budding, tillering, etc.) and
is distinguished from "apomixis", which is a "replacement" for sexual reproduction, and
in some cases involves seeds. Apomixis occurs in many plant species and also in some
non-plant organisms.
Many plants have evolved a complex sexuality, which is expressed in different
combinations of their reproductive organs. Some species have separate male and
female individuals, but the majority of plants have both male and female parts in the
same flower. Some plants change their gender expression depending on a number of
factors like age, time of day, or because of environmental conditions. Plant sexuality
also varies within different populations of some species.

ASEXUAL REPRODUCTION IN PLANTS

BINARY FISSION
Many single-celled organisms (unicellular), such as archaea, bacteria, and protists,
reproduce asexually through binary fission. An exception to the
rule are unicellular fungi such as fission yeast, unicellular algae
such as Chlamydomonas, and ciliates and some other protists,
which reproduce both sexually and asexually. Some singlecelled organisms (unicellular) rely on one or more host
organisms in order to reproduce, but most literally divide into two
organisms.
BUDDING
Some cells split via budding (for example baker's yeast), resulting in
a 'mother' and 'daughter' cell. The offspring organism is smaller than
the parent. Budding is also known on a multicellular level; an animal
example is the hydra, which reproduces by budding. The buds grow
into fully matured individuals which eventually break away from the
parent organism.
VEGETATIVE REPRODUCTION
Vegetative reproduction is a type of asexual reproduction found in
plants where new individuals are formed without the production of
seeds or spores by meiosis or syngamy. Examples of vegetative
reproduction include the formation of miniaturized plants called
plantlets on specialized leaves (for example in kalanchoe), and
some produce new plants out of rhizomes or stolon (for example in
strawberry). Other plants reproduce by forming bulbs or tubers (for
example tulip bulbs and dahlia tubers). Some plants produce adventitious shoots and

suckers that form along their lateral roots. Plants that reproduce vegetatively may form
a clonal colony, where all the individuals are clones, and the clones may cover a large
area.

SPORE FORMATION
Many multicellular organisms form spores during their biological life cycle in a process
called sporogenesis. Exceptions are animals and some protists, who undergo gametic
meiosis immediately followed by fertilization. Plants and many algae on the other hand
undergo sporic meiosis where meiosis leads to the formation of haploid spores rather
than gametes. These spores grow into multicellular individuals (called gametophytes in
the case of plants) without a fertilization event. These haploid individuals give rise to
gametes through mitosis. Meiosis and gamete formation therefore occur in separate
generations or "phases" of the life cycle, referred to as alternation of generations. Since
sexual reproduction is often more narrowly defined as the fusion of gametes
(fertilization), spore formation in plant sporophytes and algae might be considered a
form of asexual reproduction (agamogenesis) despite being the result of meiosis and
undergoing a reduction in ploidy. However, both events (spore formation and
fertilization) are necessary to complete sexual reproduction in the plant life cycle.

Fungi and some algae can also utilize true asexual spore formation, which involves
mitosis giving rise to reproductive cells called mitospores that develop into a new

organism after dispersal. This method of reproduction is found for example in conidial
fungi and the red alga Polysiphonia, and involves sporogenesis without meiosis. Thus
the chromosome number of the spore cell is the same as that of the parent producing
the spores. However, mitotic sporogenesis is an exception and most spores, such as
those of plants, most Basidiomycota, and many algae, are produced by meiosis.
FRAGMENTATION
Fragmentation is a form of asexual reproduction where a new
organism grows from a fragment of the parent. Each fragment
develops into a mature, fully grown individual. Fragmentation is
seen in many organisms such as animals (some annelid worms
and sea stars), fungi, and plants. Some plants have specialized
structures for reproduction via fragmentation, such as gemmae
in liverworts. Most lichens, which are a symbiotic union of a
fungus and photosynthetic algae or bacteria, reproduce through
fragmentation to ensure that new individuals contain both
symbionts. These fragments can take the form of soredia, dustlike particles consisting of fungal hyphae wrapped around photobiont cells.
PARTHENOGENESIS
Parthenogenesis is a form of agamogenesis in which an
unfertilized egg develops into a new individual.
Parthenogenesis occurs naturally in many plants. In
plants, apomixis may or may not involve
parthenogenesis.

SEXUAL REPRODUCTION IN PLANTS

Flowering plants
Flowering plants are the dominant plant form on land and they reproduce by
sexual and asexual means. Often their most distinguishing feature is their reproductive
organs, commonly called flowers. The anther produces male gametophytes, the sperm
is produced in pollen grains, which attach to the stigma on top of a carpel, in which the
female gametophytes (inside ovules) are located. After the pollen tube grows through
the carpel's style, the sex cell nuclei from the pollen grain migrate into the ovule to
fertilize the egg cell and endosperm nuclei within the female gametophyte in a process
termed double fertilization. The resulting zygote develops into an embryo, while the
triploid endosperm (one sperm cell plus two female cells) and female tissues of the
ovule give rise to the surrounding tissues in the developing seed. The ovary, which
produced the female gametophyte(s), then grows into a fruit, which surrounds the
seed(s). Plants may either self-pollinate or cross-pollinate. Nonflowering plants like
ferns, moss and liverworts use other means of sexual reproduction.

Flowers are the sexual organs of flowering plants.

DOUBLE FERTILIZATION
Double fertilization is a complex fertilization mechanism that has evolved in
flowering plants, known as angiosperms. This process involves the joining of a female
gametophyte (embryo sac) with two male gametes (sperm). It begins when a pollen
grain adheres to the stigma of the carpel, the female reproductive structure of a flower.
After a pollen grain has landed on an accessible stigma, the pollen grain takes in
moisture and begins to germinate, forming a pollen tube that extends down toward the
ovary through the style. The tip of the pollen tube then enters the ovary and penetrates
through the micropyle. The micropyle is an opening in the protective layers of the ovule.

The pollen tube proceeds to release the two sperm in or near the embryo sac.

One sperm fertilizes the egg cell and the other sperm combines with the two polar
nuclei of the large central cell of the embryo sac. The sperm and haploid egg combine
to form a diploid zygote, while the other sperm and two haploid polar nuclei form a
triploid nucleus (some plants may form polyploid nuclei). The large cell of the embryo
sac will then form the endosperm, a nutrient-rich tissue which provides nourishment to
the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which
is used for protection and dispersion of the seeds.

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm arise
by mitosis from a single meiotic product. Therefore, maternal contribution to the genetic
constitution of the triploid endosperm is different from that of the embryo.
In a recent study done of the plant Arabidopsis thaliana, the migration of male nuclei
inside the female gamete, in fusion with the female nuclei, has been documented for the
first time using in vivo imaging. Identification of the genes involved in the migration and
fusion process has also been determined.