Parts of plant enabling sexual reproduction
Plant reproductive morphology
is the study of the physical form and structure (the
morphology
) of those parts of plants directly or indirectly concerned with
sexual reproduction
.
Among all living organisms,
flowers
, which are the reproductive structures of
angiosperms
, are the most varied physically and show a correspondingly great diversity in methods of reproduction.
[1]
Plants that are not flowering plants (
green algae
,
mosses
,
liverworts
,
hornworts
,
ferns
and
gymnosperms
such as
conifers
) also have complex interplays between morphological adaptation and environmental factors in their sexual reproduction. The breeding system, or how the
sperm
from one plant fertilizes the
ovum
of another, depends on the reproductive morphology, and is the single most important determinant of the genetic structure of nonclonal plant populations.
Christian Konrad Sprengel
(1793) studied the reproduction of flowering plants and for the first time it was understood that the
pollination
process involved both
biotic
and
abiotic
interactions.
Charles Darwin
's theories of
natural selection
utilized this work to build his
theory of evolution
, which includes analysis of the
coevolution
of flowers and their
insect
pollinators
.
Use of sexual terminology
[
edit
]
Plants have complex lifecycles involving
alternation of generations
. One generation, the
sporophyte
, gives rise to the next generation, the
gametophyte
asexually via
spores
. Spores may be identical isospores or come in different sizes (
microspores
and
megaspores
), but strictly speaking, spores and sporophytes are neither male nor female because they do not produce
gametes
. The alternate generation, the gametophyte, produces gametes,
eggs
and/or
sperm
. A gametophyte can be
monoicous
(bisexual), producing both eggs and sperm, or dioicous (unisexual), either female (producing eggs) or male (producing sperm).
In the
bryophytes
(
liverworts
,
mosses
, and
hornworts
), the sexual gametophyte is the dominant generation. In
ferns
and
seed plants
(including
cycads
,
conifers
,
flowering plants
, etc.) the sporophyte is the dominant generation; the obvious visible plant, whether a small herb or a large tree, is the sporophyte, and the gametophyte is very small. In bryophytes and ferns, the gametophytes are independent, free-living plants, while in seed plants, each female megagametophyte, and the megaspore that gives rise to it, is hidden within the sporophyte and is entirely dependent on it for nutrition. Each male gametophyte typically consists of two to four cells enclosed within the protective wall of a pollen grain.
The sporophyte of a flowering plant is often described using sexual terms (e.g. "female" or "male")
based on the sexuality of the gametophyte it gives rise to
. For example, a sporophyte that produces spores that give rise only to male gametophytes may be described as "male", even though the sporophyte itself is asexual, producing only spores. Similarly, flowers produced by the sporophyte may be described as "unisexual" or "bisexual", meaning that they give rise to either one sex of gametophyte or both sexes of the gametophyte.
[2]
[
page needed
]
Flowering plants
[
edit
]
Basic flower morphology
[
edit
]
The
flower
is the characteristic structure concerned with sexual reproduction in flowering plants (angiosperms). Flowers vary enormously in their structure (morphology). A
perfect
flower, like that of
Ranunculus glaberrimus
shown in the figure, has a
calyx
of outer
sepals
and a
corolla
of inner
petals
and both male and female sex organs. The sepals and petals together form the
perianth
. Next inwards there are numerous
stamens
, which produce
pollen
grains, each containing a microscopic male gametophyte. Stamens may be called the "male" parts of a flower and collectively form the androecium. Finally in the middle there are
carpels
, which at maturity contain one or more
ovules
, and within each ovule is a tiny female gametophyte.
Carpels may be called the "female" parts of a flower and collectively form the gynoecium.
Each carpel in
Ranunculus
species is an
achene
that produces one ovule,
[4]
which when fertilized becomes a seed. If the carpel contains more than one seed, as in
Eranthis hyemalis
, it is called a
follicle
. Two or more carpels may be fused together to varying degrees and the entire structure, including the fused styles and stigmas may be called a
pistil
. The lower part of the pistil, where the ovules are produced, is called the
ovary
. It may be divided into chambers (
locules
) corresponding to the separate carpels.
Variations
[
edit
]
A perfect flower has both stamens and carpels, and is described as "bisexual" or "hermaphroditic". A unisexual flower is one in which either the stamens or the carpels are missing,
vestigial
or otherwise non-functional. Each flower is either staminate (having only functional stamens and thus male), or carpellate or pistillate (having only functional carpels and thus female). If separate staminate and carpellate flowers are always found on the same plant, the species is described as
monoecious
. If separate staminate and carpellate flowers are always found on different plants, the species is described as
dioecious
.
[6]
A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species.
[7]
Members of the birch family (
Betulaceae
) are examples of monoecious plants with unisexual flowers. A mature alder tree (
Alnus
species) produces long catkins containing only male flowers, each with four stamens and a minute perianth, and separate stalked groups of female flowers, each without a perianth.
(See the illustration of
Alnus serrulata
.)
Most hollies (members of the genus
Ilex
) are dioecious. Each plant produces either functionally male flowers or functionally female flowers. In
Ilex aquifolium
(see the illustration), the common European holly, both kinds of flower have four sepals and four white petals; male flowers have four stamens, female flowers usually have four non-functional reduced stamens and a four-celled ovary.
Since only female plants are able to set fruit and produce berries, this has consequences for gardeners.
Amborella
represents the first known group of flowering plants to separate from their common ancestor. It too is dioecious; at any one time, each plant produces either flowers with functional stamens but no carpels, or flowers with a few non-functional stamens and a number of fully functional carpels. However,
Amborella
plants may change their "sex" over time. In one study, five cuttings from a male plant produced only male flowers when they first flowered, but at their second flowering three switched to producing female flowers.
[10]
In extreme cases, almost all of the parts present in a complete flower may be missing, so long as at least one carpel or one stamen is present. This situation is reached in the female flowers of duckweeds (
Lemna
), which consist of a single carpel, and in the male flowers of spurges (
Euphorbia
) which consist of a single stamen.
A species such as
Fraxinus excelsior
, the common ash of Europe, demonstrates one possible kind of variation. Ash flowers are wind-pollinated and lack petals and sepals. Structurally, the flowers may be bisexual, consisting of two stamens and an ovary, or may be male (staminate), lacking a functional ovary, or female (carpellate), lacking functional stamens. Different forms may occur on the same tree, or on different trees.
The Asteraceae (sunflower family), with close to 22,000 species worldwide, have highly modified inflorescences made up of flowers (florets) collected together into tightly packed heads. Heads may have florets of one sexual morphology ? all bisexual, all carpellate or all staminate (when they are called
homogamous
), or may have mixtures of two or more sexual forms (heterogamous).
[12]
Thus goatsbeards (
Tragopogon
species) have heads of bisexual florets, like other members of the tribe Cichorieae,
[13]
whereas marigolds (
Calendula
species) generally have heads with the outer florets bisexual and the inner florets staminate (male).
[14]
Like
Amborella
, some plants undergo sex-switching. For example,
Arisaema triphyllum
(Jack-in-the-pulpit) expresses sexual differences at different stages of growth: smaller plants produce all or mostly male flowers; as plants grow larger over the years the male flowers are replaced by more female flowers on the same plant.
Arisaema triphyllum
thus covers a multitude of sexual conditions in its lifetime: nonsexual juvenile plants, young plants that are all male, larger plants with a mix of both male and female flowers, and large plants that have mostly female flowers.
[15]
Other plant populations have plants that produce more male flowers early in the year and as plants bloom later in the growing season they produce more female flowers.
[
citation needed
]
Terminology
[
edit
]
The complexity of the morphology of flowers and its variation within populations has led to a rich terminology.
- Androdioecious
: having male flowers on some plants, bisexual ones on others.
[6]
- Androecious
: having only male flowers (the male of a
dioecious
population); producing pollen but no seed.
[16]
- Androgynous
: see
bisexual
.
[6]
- Androgynomonoecious
: having male, female, and bisexual flowers on the same plant, also called trimonoecious.
[16]
- Andromonoecious
: having both
bisexual
and male flowers on the same plant.
[6]
- Bisexual
: each flower of each individual has both male and female structures, i.e. it combines both sexes in one structure.
[6]
Flowers of this kind are called
perfect
, having both
stamens
and
carpels
. Other terms used for this condition are
androgynous
,
hermaphroditic
,
monoclinous
and
synoecious
.
- Dichogamous
: having sexes developing at different times; producing pollen when the stigmas are not receptive,
[6]
either
protandrous
or
protogynous
. This promotes
outcrossing
by limiting self-pollination.
[17]
Some dichogamous plants have
bisexual
flowers, others have
unisexual
flowers.
- Diclinous
: see
Unisexual
.
[6]
- Dioecious
: having either only male or only female flowers.
[6]
No individual plant of the population produces both pollen and ovules.
[18]
(From the Greek for "two households". See also the Wiktionary entry for
dioecious
.)
- Gynodioecious
: having hermaphrodite flowers and female flowers on separate plants.
[19]
- Gynoecious
: having only female flowers (the female of a
dioecious
population); producing seed but not pollen.
[20]
- Gynomonoecious
: having both
bisexual
and female flowers on the same plant.
[6]
- Hermaphroditic
: see
bisexual
.
[6]
- Homogamous:
male and female sexes reach maturity in synchrony; producing mature pollens when stigma is receptive.
- Imperfect
: (of flowers) having some parts that are normally present not developed,
e.g. lacking stamens. See also
Unisexual
.
- Monoclinous
: see
bisexual
.
[6]
- Monoecious
: In the commoner narrow sense of the term, it refers to plants with
unisexual
flowers which occur on the same individual.
[2]
In the broad sense of the term, it also includes plants with
bisexual
flowers.
[6]
Individuals bearing separate flowers of both sexes at the same time are called simultaneously or synchronously monoecious and individuals that bear flowers of one sex at one time are called consecutively monoecious.
[22]
(From the Greek
monos
"single" +
oikia
"house". See also the Wiktionary entry for
monoecious
.)
- Perfect
: (of flowers) see
bisexual
.
[6]
- Polygamodioecious
: mostly
dioecious
, but with either a few flowers of the opposite sex or a few
bisexual
flowers on the same plant.
[2]
- Polygamomonoecious
: see
polygamous
.
[6]
Or, mostly monoecious, but also partly polygamous.
[2]
- Polygamous
: having male, female, and
bisexual
flowers on the same plant.
[6]
Also called
polygamomonoecious
or
trimonoecious
.
[23]
Or, with bisexual and at least one of male and female flowers on the same plant.
[2]
- Protandrous
: (of
dichogamous
plants) having male parts of flowers developed before female parts, e.g. having flowers that function first as male and then change to female or producing pollen before the stigmas of the same plant are receptive.
[6]
(
Protoandrous
is also used.)
- Protogynous
: (of
dichogamous
plants) having female parts of flowers developed before male parts, e.g. having flowers that function first as female and then change to male or producing pollen after the stigmas of the same plant are receptive.
[6]
- Subandroecious
: having mostly male flowers, with a few female or
bisexual
flowers.
[24]
- Subdioecious
: having some individuals in otherwise
dioecious
populations with flowers that are not clearly male or female. The population produces normally male or female plants with
unisexual
flowers, but some plants may have
bisexual
flowers, some both male and female flowers, and others some combination thereof, such as female and bisexual flowers. The condition is thought to represent a transition between bisexuality and
dioecy
.
[25]
[26]
- Subgynoecious
: having mostly female flowers, with a few male or
bisexual
flowers.
[
citation needed
]
- Synoecious
: see
bisexual
.
[6]
- Trimonoecious
: see
polygamous
[6]
and
androgynomonoecious
.
[16]
- Trioecious
: with male, female and bisexual flowers on different plants.
[27]
- Unisexual
: having either functionally male or functionally female flowers.
[6]
This condition is also called
diclinous
,
incomplete
or
imperfect
.
Outcrossing
[
edit
]
Outcrossing
, cross-fertilization or allogamy, in which offspring are formed by the fusion of the
gametes
of two different plants, is the most common mode of reproduction among
higher plants
. About 55% of higher plant species reproduce in this way. An additional 7% are partially cross-fertilizing and partially self-fertilizing (autogamy). About 15% produce gametes but are principally self-fertilizing with significant out-crossing lacking. Only about 8% of higher plant species reproduce exclusively by non-sexual means. These include plants that reproduce vegetatively by runners or bulbils, or which produce seeds without embryo fertilization (
apomixis
). The selective advantage of outcrossing appears to be the masking of deleterious recessive mutations.
[28]
The primary mechanism used by flowering plants to ensure outcrossing involves a genetic mechanism known as
self-incompatibility
. Various aspects of floral morphology promote allogamy. In plants with bisexual flowers, the anthers and carpels may mature at different times, plants being
protandrous
(with the anthers maturing first) or protogynous (with the carpels mature first).
[
citation needed
]
Monoecious species, with unisexual flowers on the same plant, may produce male and female flowers at different times.
[
citation needed
]
Dioecy, the condition of having unisexual flowers on different plants, necessarily results in outcrossing, and probably evolved for this purpose. However, "dioecy has proven difficult to explain simply as an outbreeding mechanism in plants that lack self-incompatibility".
[7]
Resource-allocation constraints may be important in the evolution of dioecy, for example, with wind-pollination, separate male flowers arranged in a catkin that vibrates in the wind may provide better pollen dispersal.
[7]
In climbing plants, rapid upward growth may be essential, and resource allocation to fruit production may be incompatible with rapid growth, thus giving an advantage to delayed production of female flowers.
[7]
Dioecy has evolved separately in many different lineages, and monoecy in the plant lineage correlates with the evolution of dioecy, suggesting that dioecy can evolve more readily from plants that already produce separate male and female flowers.
[7]
See also
[
edit
]
References
[
edit
]
Citations
[
edit
]
- ^
Barrett, S.C.H. (2002).
"The evolution of plant sexual diversity"
(PDF)
.
Nature Reviews Genetics
.
3
(4): 274?284.
doi
:
10.1038/nrg776
.
PMID
11967552
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7424193
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- ^
a
b
c
d
e
Hickey, M. & King, C. (2001).
The Cambridge Illustrated Glossary of Botanical Terms
. Cambridge University Press.
- ^
Whittemore, Alan T.
"
Ranunculus
"
.
Flora of North America
. Retrieved
2013-03-04
– via www.eFloras.org.
- ^
a
b
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f
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i
j
k
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m
n
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p
q
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s
t
Beentje, Henk (2010).
The Kew Plant Glossary
. Richmond, Surrey:
Royal Botanic Gardens, Kew
.
ISBN
978-1-84246-422-9
.
- ^
a
b
c
d
e
Renner, S.S. & Ricklefs, R.E. (1995).
"Dioecy and its correlates in the flowering plants"
.
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82
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10.2307/2445418
.
JSTOR
2445418
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- ^
Buzgo, Matyas;
Soltis, Pamela S.
& Soltis, Douglas E. (2004). "Floral Developmental Morphology of
Amborella trichopoda
(Amborellaceae)".
International Journal of Plant Sciences
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165
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10.1086/424024
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- ^
Barkley, Theodore M.; Brouillet, Luc & Strother, John L.
"Asteraceae"
.
Flora of North America
. Retrieved
2013-03-04
– via www.eFloras.org.
- ^
Barkley, Theodore M.; Brouillet, Luc & Strother, John L.
"Chichorieae"
.
Flora of North America
. Retrieved
2013-03-04
– via www.eFloras.org.
- ^
Strother, John L.
"
Calendula
"
.
Flora of North America
. Retrieved
2013-03-04
– via www.eFloras.org.
- ^
Ewing, J.W. & Klein, R.M. (1982). "Sex Expression in Jack-in-the-Pulpit".
Bulletin of the Torrey Botanical Club
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109
(1): 47?50.
doi
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10.2307/2484467
.
JSTOR
2484467
.
- ^
a
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Janick, J. (2010).
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. Wiley.
ISBN
9780470650028
.
- ^
Stace, H.M. (1995). "Protogyny, Self-Incompatibility and Pollination in
Anthocercis gracilis
(Solanaceae)".
Australian Journal of Botany
.
43
(5): 451?459.
doi
:
10.1071/BT9950451
.
- ^
Baskauf, Steve (2002).
"Sexual systems in angiosperms"
. Archived from
the original
on 2018-07-03
. Retrieved
2013-02-27
.
- ^
"Gynodioecious"
. Dictionary of Botany
. Retrieved
2013-04-10
.
- ^
G. J. H. Grubben (2004).
Vegetables
. PROTA. pp.
255
?.
ISBN
978-90-5782-147-9
.
- ^
Dinesh Kumar (20 August 2008).
Definitional Glossary of Agricultural Terms
. I. K. International Pvt Ltd. pp. 115?.
ISBN
978-81-906757-4-1
.
- ^
Geber, Monica A. (1999).
Gender and sexual dimorphism in flowering plants
. Berlin: Springer.
ISBN
3-540-64597-7
.
p. 4
- ^
Testolin, R; Cipriani, G; Costa, G (May 1995).
"Sex segregation ratio and gender expression in the genus Actinidia"
.
Sexual Plant Reproduction
.
8
(3).
doi
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10.1007/BF00242255
.
S2CID
25414438
. Retrieved
30 December
2020
.
- ^
Olson, Matthew S. & Antonovics, Janis (2000). "Correlation between male and female reproduction in the subdioecious herb
Astilbe biternata
(Saxifragaceae)".
American Journal of Botany
.
87
(6): 837?44.
doi
:
10.2307/2656891
.
JSTOR
2656891
.
PMID
10860914
.
- ^
Strittmatter, L.I.; Negron-Ortiz, V. & Hickey, R.J. (2002). "Subdioecy in
Consolea spinosissima
(Cactaceae): breeding system and embryological studies".
American Journal of Botany
.
89
(9): 1373?1387.
doi
:
10.3732/ajb.89.9.1373
.
PMID
21665739
.
- ^
Beentje, Henk (2016).
The Kew Plant Glossary
(second ed.). Richmond, Surrey:
Royal Botanic Gardens, Kew
.
ISBN
978-1-84246-604-9
.
- ^
Bernstein, C. & Bernstein, H. (1991).
Aging, Sex, and DNA Repair
. San Diego: Academic Press.
ISBN
978-0-12-092860-6
.
Sources
[
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]
Further reading
[
edit
]
External links
[
edit
]