Functionally, the flower is the plant organ responsible for sexual reproduction. A complete or ideal flower consists of four peripheries: two lower non-essential peripheries—the calyx and the corolla—and two upper essential peripheries—the stamens and the pistil (Figure 2a-d). When the calyx and corolla are indistinguishable, they are collectively called the perianth. The perianth consists of floral parts called tepals.
The stamen is the male
perianth,
composed of the stamens. Each stamen consists of a filament, an anther, and a
connective tissue. The female perianth is the pistil, composed of the ovary,
style, and stigma.
A flower that bears both
stamens and pistils is called a bisexual or hermaphroditic flower. A
flower that bears only one of the basic perianths is called a unisexual flower.
A unisexual flower is either male if it bears only stamens or female if it
bears only carpels. Plants that bear both male and female flowers are known as
monoecious, such as fan palms, mulberries, and poppies. On the other hand,
plants that bear unisexual, hermaphroditic, and asexual flowers are known as
polygamous, such as mangoes and polygonum. Additionally, flowers lacking a
calyx and corolla are known as achlamydeous, such as betel. Finally, flowers
with only one of the non-essential perianths are known as monoecious, such as
those of the polygonum plant. Flowers containing two non-essential perianths
are known as dichlamydous flowers.
A flower is called spiral
or annular if the sepals, petals, stamens, and carpels are arranged in
rings or perianths around the thalamus, as in most flowers. It is called
non-spiral if the perianths are arranged spindle-shaped, as in magnolias,
lilies, water irises, etc. A flower may be semi-spiral if some perianths are
spiral and others are not, as in roses.
Flower receptacle
Flower receptacle shape
The receptacle (or torus) is the swollen, compressed end of the floral axis to which the floral parts—sepals, petals, stamens, and carpels—are attached (Figure 3).
The receptacle is very
short
in most flowers, but it can be elongated in some limited cases. The nodes and
internodes are clearly visible. In Silene, the internode between the calyx and
corolla is elongated and is called the anthophore (Figure 3a). In Gynandropsis
and Passion flower,
the internode between the corolla and the stamen is greatly elongated and is
known as the androphore (Figure 3b). In Drumstick, the axis between the stamen
and the pistil is elongated and is known as the gynophore (Figure 3c). If the
internodes between the corolla and the pistil and the internodes between the
stamen and the pistil are both elongated, they are known as the androgynophore,
as in Gynandropsis (Figure 3d). The shape and nature of the receptacle vary
from flower to flower. It may be fleshy and elongated, bearing the petals in a
spindle-like arrangement, as in magnolia. It may be concave and pear-shaped, as
in rose (Figure 3e), or spongy and apical, as in lotus (Figure 3f). In balsam,
anise, coriander, cumin, the genus Geranium, etc., the receptacle elongates
upwards, forming a cylindrical axis that initially supports the carpels. These
carpels then separate from it upon maturity and are known as the carpophore
(Figure 3L).
Positioning of Floral Leaves on the Receptacle: Normally, the calyx, corolla, stamens, and pistil are arranged on the receptacle in a specific sequence. However, the receptacle may grow abnormally, disrupting this arrangement. This results in variations in the relative positions of the first three peripheries (calyx, corolla, and stamens) relative to the ovary. This variation is limited to three positions: inferior, periphery, and superior (Figure 4).
Inferior Flower: In an
inferior flower, the receptacle is conical, convex, flat, or slightly concave.
The ovary occupies the highest point of the receptacle, while the other
peripheries lie below the ovary. The flower is called the inferior flower, and
the ovary is known as the superior ovary (Figure 4a). Examples of inferior
flowers include the Chinese rose, magnolia, eggplant, and mustard.
Superior flower: In this type
of flower, the receptacle is either cup or
flask-like. It differs from the perianthic flower in that the carpels fuse
together and adhere to the receptacle wall. This results in the closure of the
upper receptacle opening, burying the pistil within its cavity. The petals and
stamens emerge above the ovary. Therefore, the pistil is inferior, and the
other floral organs are superior (Figure 4b). Examples of this type of flower
include the sunflower, cucumber, apple, and pear.
Perianthic flower: In the
perianthic flower, the edge of the receptacle grows upwards, forming a cup-like
structure called the calyx tube. It surrounds the ovary but is not attached to
it. At its tip, it bears the sepals, petals, and stamens. In this case, the
ovary is referred to as the perianthic flower. Lower half (Figure 4C). In some
periwinkles, the ovary is partially submerged in the receptacle. Examples of
periwinkles include roses, apricots, dates, peas, and beans.
Bracts
Bracts are specialized leaves from whose axils one or more flowers emerge (Figure 5). Sometimes, a small, scaly, leaf-like structure known as a bracteole is present on the flower stalk. Bracts vary in shape, size, color, and age and include the following types:
Foliaceous Bracts: The
foliaceous bract is green, flat, and resembles a leaf in appearance, as in the
hollyhock (Figure 5a).
Spathe: The spathe
is a large, boat-like bract that surrounds a cluster of flowers or an
inflorescence. It is known as the spadix, as in bananas, dates, and the female
inflorescence of corn.
Petaloid bracts: Brightly
colored bracts, somewhat resembling petals, as in bougainvillea (Figure 5C). In
Poinsettia
(Euphorbia pulcherrima), the bracts
are red and leaf-shaped.
Involucre: The
involucre consists of one or more peripheries of green bracts surrounding a
cluster of flowers. The involucre distinguishes members of the Asteraceae
family, such as sunflowers, anise, carrots, and coriander (Figure 5D).
Epicalyx: The epicalyx
consists of one or more peripheries of bracts that arise at the base of the
calyx. The epicalyx distinguishes members of the Malvaceae family, such as
cotton, lady's finger, and hollyhock (Figure 5e). It is also found below the
calyx in many plants belonging to the Rosaceae family, such as strawberries.
Scale bracteole: The scale
bracteole is found at the base of each floret of the important inflorescence,
or head, that characterizes members of the Asteraceae family (Figure 5f). The
scale bracteole is a thin, membranous, drill-like structure.
Glumes: Glumes are
specialized bracts—small, dry, and scaly—found in the spikelets of grasses
(Figure 5n). They take the form of two tiny, fine scales known as the vacuoles.
The floral glumes are known as the lower bract (lemma), and the bract is known
as the inner bract (palea).
Origin of the flower: The origin
of the flower and its external and anatomical structure indicate that the
flower is a modified branch. It originated from a modified vegetative bud. The
receptacle is the axis of this branch, and the floral parts—sepals, petals,
stamens, and carpels—are leaves. Scientists have deduced this from the
following evidence:
The receptacle of the
flower consists of very dwarf nodes and internodes, whose normal growth is
completely halted. However, it may elongate in some flowers and show nodes and
internodes, as in the passion flower,
gynandropsis (Figure 3b), and Capparis. This
supports the view that the receptacle is a modified branch.
Sometimes, the receptacle
grows
enormously, extending above the floral periphery and bearing ordinary
vegetative leaves, as in some roses (Figure 6), pears, Delphinium, and
calendula (Calendula officinalis). Here, the
receptacle behaves exactly like a vegetative branch.
The sepals and leaves are
similar in structure, shape, and venation. One of them may be
modified. The sepals develop into a distinct, usually colored, or sometimes
white, leaf-like structure, as in Mussaenda (Figure 7). The origin of
the petals, however, remains a subject of debate. Some believe that petals are
related to the sepals, while others believe they originated from the stamens.
The first view is supported by the similarity of the petals of the green rose
to the leaf in color and structure. On the other hand, there is no resemblance
between the stamens and carpels on the one hand and the leaf on the other.
However, some similarities can be drawn between the leaf and both the stamens
and carpels in certain flowers. For example, there is a gradual transition from
sepals to petals and from petals to stamens in the water lily (Figure 8a). This
illustrates that sepals may gradually transform into stamens, and stamens may
also transform into petals. Some or most stamens may transform into petals, as
in some hollyhock species, such as Hibiscus mutabilis (Figure 8a). 8b)
The flower bud and the
vegetative bud are similar in position: apical or axillary.
Likewise, the arrangement of sepals, petals, etc., on the receptacle closely
resembles the arrangement of leaves on the stem or branch: parapetal,
alternate, or opposite.
The inflorescence axis
usually
bears the flowers. However, in the American cactus, some flower buds are
modified into vegetative buds, known as bulbs, which carry out vegetative
reproduction. Similarly, the pineapple inflorescence axis bears one or more
vegetative buds or bulbs (Figure 9), whose role is also limited to vegetative
reproduction.
Symmetry in the Flower: A flower is
said to be symmetrical or regular if it can be divided into two symmetrical
halves by any vertical section passing through its center, such as the flowers
of mustard, datura, eggplant, blackberry, and chili pepper (Figure 10). If a
flower can be divided into two identical halves by only one vertical plane, it
is called a monosymmetrical flower, as in the pea, broad bean, cassia,
snapdragon, and golden mohair (Delonix). If a flower cannot be divided into two
identical halves by any vertical plane, it is called an irregular flower, as in
the canna and bird of paradise.
A flower is also called
symmetrical if its peripheries (sepals, petals, and stamens) consist of
the same number of petals, or if the number in one periphery is double the
number in another. These flowers are known as isomers. Isomerous flowers may be
bipartite, tripartite, tetrapartite, or pentatonic, depending on the number of
petals in each periphery: 2, 3, 4, 5, or multiples. Bipartite and tripartite
symmetry characterize monocotyledonous flowers. Five-sided and four-sided
symmetry characterize dicotyledons. If the number of petals varies in different
peripheries, the flower is heteromorphic.
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