The Flower

In angiosperms, the flower is the reproductive unit. It's designed to be used for sexual reproduction. On the swollen end of the stalk or pedicel, called the thalamus or receptacle, a typical flower has four different kinds of whorls placed sequentially. Calyx, corolla, androecium, and gynoecium are the four flower parts. Androecium and gynoecium are reproductive organs, while calyx and corolla are accessory organs. The calyx and corolla of some flowers, such as lilies, are not distinct and are referred to as perianth. A flower is bisexual if it has both androecium and gynoecium. Unisexual flowers are those that have only stamens or carpels.

The flower can be actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry). Flowers that have multiple lines of symmetry (like a starfish) are radially symmetrical, also called actinomorphic. Flowers with only a single line of symmetry are bilaterally symmetrical, also called zygomorphic.  Actinomorphic flowers, such as mustard, datura, and chilli, can be divided into two equal radial halves in any radial plane running through the centre. It is zygomorphic when it can be separated into two comparable halves only in one vertical plane, as in pea, Gulmohar, bean, and Cassia. If a flower, like canna, cannot be divided into two comparable halves by any vertical plane going through the centre, it is asymmetric (irregular).

Figure 13: Symmetry in flowers

Trimerous, tetramerous, and pentamerous flowers have floral appendages in multiples of 3, 4, and 5, respectively. Bracteate flowers have a shortened leaf at the base of the pedicel, whereas ebracteate flowers do not have one. Flowers are classified as hypogynous, perigynous, or epigynous based on the position of the calyx, corolla, and androecium in relation to the ovary on the thalamus. The gynoecium is at the top of the hypogynous flower, with the other sections below it. Such flowers, such as mustard, China rose, and brinjal are said to have superior ovaries. If the gynoecium is in the center and the other parts of the flower are virtually at the same level as the thalamus rim, the flower is said to be perigynous.Plum, rose, and peach, for example, have a half inferior ovary. The thalamus margin develops upward in epigynous flowers, entirely enclosing and fusing with the ovary; the other parts of the flower ascend above the ovary. As a result, the ovary is reported to be inferior in guava and cucumber flowers, as well as sunflower ray florets.

Figure 14: Position of floral parts

Parts of a flower include:

Figure 15: Parts of a flower

Calyx: The calyx is the flower's outermost whorl, and the sepals are its members. Sepals are green, leaf-like structures that protect the flower at the bud stage. The calyx can be gamosepalous (sepals together) or polysepalous (sepals apart) (sepals free).

Corolla: Petals make up the corolla. Petals are typically vividly coloured to attract pollinating insects. Corollas can be gamopetalous (petals joined) or polypetalous, just as the calyx (petals free). Plants have a wide range of corolla shapes and colours. Tubular, bell-shaped, funnel-shaped, or wheel-shaped corollas are all possible.

Aestivation refers to the way sepals or petals in a floral bud are arranged in relation to the other members of the same whorl. Valvate, twisted, imbricate, and vexillary aestivation are the most common kinds. Valvate describes a whorl in which the sepals or petals only touch at the margin, without overlapping, as in Calotropis. Twisted means that one appendage's edge overlaps the next and so on, as in the china rose, lady's finger, and cotton. Imbricate aestivation occurs when the borders of sepals or petals overlap but not in any specific direction, as in Cassia and Gulmohar.The largest (standard) petal overlaps the two lateral petals (wings), which overlap the two smallest anterior petals (keel) in pea and bean flowers; this type of aestivation is known as vexillary or papilionaceous.

   Figure 16: Types of aestivation
   Figure 17: Classification of aestivation.

Androecium: Stamens make up the androecium. A stalk or filament and an anther make up each stamen, which represents the male reproductive organ. Each anther is normally bilobed, with two pollen-sac chambers in each lobe. Pollen grains are manufactured in pollen sacs. The staminode is a sterile stamen. Flower stamens can be joined to other components of the flower, such as petals, or to each other. Stamens are epipetalous when attached to the petals, as in brinjal, or epiphyllous when attached to the perianth, as in lily flowers. A flower's stamens can either remain free (polyandrous) or be joined to varying degrees.The stamens can be grouped into a single bunch or bundle (monoadelphous), two bundles (diadelphous), or more than two bundles (polyadelphous) as in China rose. Within a flower, filament length can vary, as in the case of Salvia and mustard.

Gynoecium: The female reproductive component of the flower is the gynoecium, which is made up of one or more carpels. The stigma, style, and ovary are the three sections of a carpel. The expanded basal portion of the ovary is where the elongated tube, the style, is located. The ovary is linked to the stigma by the style. The stigma is the receptive surface for pollen grains and is usually found at the tip of the style. One or more ovules are linked to a flattened, cushion-like placenta in each ovary. When more than one carpel is present, it is referred to as apocarpous as in lotus and rose. When carpels fuse, as in mustard and tomato, they are called syncarpous.The ovules develop into seeds after fertilization, and the ovary matures into a fruit.

Placentation is defined as the placement of ovules within the ovary. There are several varieties of placentation: marginal, axile, parietal, basal, central, and free central. The placenta forms a ridge along the ventral suture of the ovary in marginal placentation, and the ovules are carried in two rows on this ridge, as in pea. The placenta is considered to be axile when the placenta is axial and the ovules are linked to it in a multilocular ovary, as in the China rose, tomato, and lemon. The ovules develop on the inner wall of the ovary or on the peripheral region in parietal placentation. In mustard and Argemone, the ovary is one-chambered but becomes two-chambered due to the creation of the false septum.The placentation is called free central when the ovules are borne on the central axis and the septa are missing, as in Dianthus and Primrose. As in sunflower and marigold, the placenta develops at the base of the ovary and is linked to a single ovule.

Figure 18: Types of placentation in plants.