The Androecium (Androecium)
Anatomy of the Androecium
The anther is composed of a number of stamens, or microsporophylls. Each stamen consists of a filament, an anther, and a connector (Figure 2). The anther is composed of two lobes, each lobe of which has two chambers, called pollen sacs or microsporangia. Therefore, each anther contains four chambers (Figure 2), filled with pollen grains or microspores. In some cases, the anther may contain one or two chambers.
Comparing the anther to a leaf, the
filament is analogous to the petiole. The anther is analogous to the blade. The
connector is analogous to the midrib. The connector is attached to the dorsum
of the anther, and the opposite side represents the face of the anther. The
face has a longitudinal groove that extends from its apex to its base. If the
anther faces towards the pistil, it is called an introrse anther. If it faces
outwards, it is called an extrose anther. In some flowers, some anthers may
face toward the pistil and others outward, as in buttercups and cinnamon.
Stamens without pollen grains are called sterile stamens, as in carnations and Pentapetes
(Figure 1).
- Filament-Anther Connection
There are five main ways in which the
filament connects to the anther. The anther is called basal-fixed if the
filament connects to the base of the anther, as in mustard and radishes. It is
called dorsifixed if it connects to the anther from the back, as in passion
flowers. It is known as mobile if it connects to the anther from the back at
only one point. Therefore, it swings freely in the air, as in the grass family
(Poaceae). It is known as "broad-connected" (adnate) if the anther
connects to the filament along its entire length, as in magnolia. The
connective may grow and elongate, separating the two lobes, as in the salvia
flower. The stamen then transforms into a first-class lever bearing one of the
anther lobes at each end (Figure 2).
- Cohesion and Adhesion
The term "cohesion" is used
to refer to the adhesion of different parts of the periphery, such as petals
with stamens or stamens with carpels. The term "adhesion" is used to
refer to the joining of parts of the same periphery, such as stamens with each
other or carpels with each other.
Stamen Adhesion
The stamens may be separate or fused.
Stamens are joined to varying degrees. The union may be "adelphous"
if only the filaments of the anthers are joined together. It may be
"syngenesious" if only the anthers are joined together. And it may be
"synandrous" if both the filaments and anthers are joined. The
following types of union exist in plants:
Monoadelphous Stamens: When all the filaments are joined
into a single bundle, and the anthers remain separate, the stamens are called
monoadelphous, as in the Malvaceae family, such as cotton.
Diadelphous Stamens: When the filaments are joined into
two bundles, as in peas and beans.
Polyadelphous stamens: If the filaments are joined in more
than one bundle, as in castor beans and lemons.
Syngenesious stamens: If the anthers are joined together
in a tube and the filaments remain free, as in sunflowers.
Synandrous stamens: If the stamens are completely
joined, anthers and filaments, as in pumpkins and taro.
Stamen Cohesion
Stamens are called epipetalous if
they are attached to the corolla completely or partially via their filaments, as
in potatoes and sunflowers. They are called epiphyllous if they are attached to
the perianth, as in the lily family. It is known as "gynandrous" if
it is completely united with the carpels or if only its anthers are united, as
in orchids and calotropis.
Length of Stamens
In members of the Lamiaceae family,
there are four stamens: two long and two short. Such stamens are known as
"didynamous," as in basil. In the Brassicaceae family, there are six
stamens: the four inner ones are long, and the two outer ones are short. Such
stamens are known as "tetradynamous," as in radishes and turnips.
Sometimes the same plant bears different types of flowers, some with long
stamens and others with short stamens. This condition is known as dimorphic
stamens.
Another dehiscence
The anther may open longitudinally as
in cotton and datura, transversely as in basil, porously as in potatoes and
eggplants, or valvularly as in cumin and camphor.
Gynoecium
The gynoecium (plural of gynoecium)
consists of one or more carpels. It may be simple, as in cotton and datura, or
porous, as in potatoes and eggplant, or valve-like, as in cumin and camphor.
The gynoecium is composed of one or more carpels. It may consist of a single
carpel, in which case it is called a simple gynoecium. It may consist of two or
more carpels, in which case it is called a compound gynoecium. A compound
gynoecium may be apocarpous, as in lotus and rose, or syncarpous, as in
watermelon and zucchini.
The gynoecium consists of three
parts: the stigma, the style, and the ovary. The ovary contains one or more
egg-like bodies, oval or nearly round, representing the seed primordia, known
as ovules. Each ovule contains a large oval cell called the embryo sac.
Sometimes the pistil is sterile. The style is attached to the ovary. The
attachment may be terminal, lateral, or basal. The basal style arises from a
central depression in the middle of a four-lobed ovary. It appears to emerge
directly from the base of the ovary, as in the Lamiaceae family.
The ovary
The ovary—the carpel—is a modified
leaf. Its leaf-like nature is evident in the flowers of peas and beans, where
the flower contains only one carpel. This carpel resembles a leaf that has
folded along its midrib, its edges meeting and merging to form a single
chamber. The area of fusion is known as the ventral suture, and the midrib
along which it folds is known as the dorsal suture. On the ventral side, a
projection of tissue bearing the ovules, known as the placenta, develops. The
ovary with separate carpels develops in the manner described above. In the
ovary with fused carpels, the carpels join only at their edges, forming a
single-chambered ovary, as in orchids and poppies. Alternatively, they may fold
inwards, their edges meeting at the center, forming a multi-chambered ovary
with a central axis, as in irises and Chinese roses. In gymnosperms, the
carpels remain flat. The ovules develop naked along the open edges of the
carpel. Consequently, there is no stigma, style, or ovary.
Fused carpel ovary
In fused carpel pistils, it is
difficult to determine the number of carpels. The number of carpels in a fused
ovary can be inferred from the number of stigmas, stigma lobes, styles, ovarian
lobes, ovarian chambers, placentas in the ovary, or ovule groups in the ovary.
Carpels may be fused along their
entire length, as in most fused pistils, or they may be fused only in a
specific region of the pistil. Some carpels are fused only in the ovary region,
such as those of carnation and flax. Others are fused in both the ovary and style
regions, such as those of cotton. Some are fused in the pen and stigma region,
like the oleander's crab. Others are fused in the stigma region. Some are
partially fused in the style region, as in the "pregnant" plant.
Placentation
The placenta is a projection of
parenchyma tissue in the inner wall of the ovary to which the ova are attached.
Placens usually originate on the edges of the carpels, either along the line of
fusion or at the base or apex. The way the placentas are distributed in the
ovarian lumen is known as the placental position. In a simple ovary, there is
only one type of placental position, known as the "marginal placental
position." In a compound ovary, the placental position can be axial,
parietal, central, basal, or apical.
Axial placentation: In the axial placentation, the ovary
consists of two or more chambers. The placenta originates from the central
axis, which represents the meeting point of the carpel edges, as in lemons,
oranges, yarrow, tomatoes, and potatoes (Figure 3b).
Parietal placentation: In the parietal placentation, the
ovary consists of a single chamber. The ovule-bearing placenta originates from
the inner wall of the ovary, the point where the carpel edges meet, as in
poppies and squills. Although the ovary in cruciferous plants, such as mustard
and radishes, consists of two chambers, the placental position is also
parietal. However, it is noteworthy that the ovary initially consists of a
single chamber, which is soon divided into two by a pseudo-septum. It is also
noteworthy that the seeds remain attached to a fibrous framework or structure
known as the replum (Figure 3c).
Central placentation In central placentation, the septal
walls of young ovaries dissolve immediately, resulting in a single chamber.
Remnants of these walls may be observed in the mature ovary. The placenta,
which carries the ova, originates from a central axis, as in the carnation.
Free central placentation: In the free central placentation,
the placenta originates from the base of the ovary. It extends a considerable
distance within the ovarian lumen as a fatty or swollen central axis. It
carries the ova on its periphery or its entire surface, as in the primrose.
Because the placenta is free within the ovarian lumen, this placentation is
known as free central placentation (Figure 3d).
Basal placentation: In the basal placentation, the
ovary consists of a single chamber. The placenta originates directly on the
receptacle, carrying a single ovum at the base of the ovary, as in the
sunflower (Figure 3e).
Apical placentation: In the apical placentation, the
ovary consists of a single chamber. The placenta originates on the The
receptacle is directly attached, bearing a single ovule at its apex (Figure
3w).
- Ovule Structure
The ovule is connected to the
placenta by a cylindrical stalk known as the funicle. The point of attachment
of the ovule body to its stalk, or the funicle, is known as the hilum. In an
inverted ovule, the funicle extends beyond the hilum along the length of the
ovule body, forming a projection known as the raphe (Figure 4). The upper end
of the raphe forms the junction of the ovule and nucellus integuments, known as
the chalaza. The nucellus is surrounded by two integuments. However, in some families
with separate petals, there is only one integument. In parasitic plants, such
as sandalwood and lauryl, the integuments are entirely absent. At the apex of
the integuments is a small opening known as the micropyle. Finally, there is At
the tip of the ovule, near the micropyle, lies a large egg cell known as the
embryo sac. This sac carries the embryo and is considered the most important
part of the ovum.
Ovule Shapes
Orthotropous: An orthotropous ovule is a straight,
upright ovule where the micropyle is at the top, aligned with the chalaza and
funicle. This type is the simplest and most primitive type of ovule, also known
as the straight or erect ovule, and is found in plants like gymnosperms and
some flowering plants such as piper and polygonum. (Figure 5a).
Anatropous ovule: An anatropous ovule is a plant ovule
that is inverted 180 degrees so its opening (micropyle) faces downwards, close
to where it attaches to the stalk (funiculus). This is the most common type of
ovule, found in about 80% of angiosperms, and its orientation helps efficiently
guide the pollen tube to the egg cell for fertilization. (Figure 5b).
Amphitropous ovule: An amphitropous ovule describes an
ovule that is partially inverted, bent at a right angle on its stalk (funicle),
so the body and embryo sac are both curved. This results in the stalk being
attached near the middle of one side, making it appear somewhat
horseshoe-shaped, as in poppies and Sagittaria (Figure 5c).
Campylotropous ovule: A campylotropous ovule is a type of
ovule that is bent to one side, so the micropyle (the opening) is close to the
funiculus (the stalk). Unlike an anatropous ovule, which is completely
inverted, the campylotropous ovule's body is only partially curved, like a
horseshoe, such as in peas and families such as Brassicaceae (mustard family)
and Fabaceae (legume family) (Figure 5d).
Heminanatropous ovule: A hemin or hemitropous ovule is one
where the ovule body is bent at a 90-degree angle to the funiculus (stalk),
positioning the ovule horizontally. In this type, the micropyle and chalaza are
in a straight line, but the micropyle is away from the hilum (the point of
attachment to the stalk), as in Ranunculus and Primula (Figure 5e).
- Position of the ovule in the ovary
The ovule may be superior, meaning it
points upwards as in the sunflower. Or pendulum, meaning it hangs downwards
from the top, as in anise and coriander. Or lateral, meaning it emerges from
the side and points downwards at an oblique angle. Or horizontal, meaning it
moves from the side inwards horizontally.
References
Chinese
Academy of Sciences. 2018. "Flowers originated 50
million years earlier than previously thought".
Eames AJ.
1961. Morphology of the Angiosperms. New York:
McGraw-Hill Book Co.
Greyson
RI.1994. The Development of Flowers. Oxford University
Press.
Leins P,
Erbar C.2010. Flower and Fruit. Stuttgart: Schweizerbart
Science Publishers. .
PrennerG.
2010. "Floral formulae updated for routine inclusion
in formal taxonomic descriptions".
Taxon. 59: 241–250.
Sattler R.
1973. Organogenesis of Flowers. A Photographic
Text-Atlas. University of Toronto Press
SattlerR.1978.
"'Fusion' and 'continuity' in floral morphology".
Notes of the Royal Botanic Garden, Edinburgh.
36: 397–405.
Sharma OP. 2009. Plant Taxonomy (2nd ed.). Tata McGraw-Hill Education. pp. 165–166.
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