The arrangement of flowers on floral axis is called inflorescence. There are two major types of inflorescence, viz. racemose and cymose.
Racemose Inflorescence: In this type of inflorescence the main axis continues to grow. Flowers are borne laterally in acropetal succession. In case of acropetal succession, the older flowers are at base and the younger flowers are at top.
Cymose inflorescence: In this type of inflorescence, the main axis terminates in a flower. Hence, the growth is limited in cymose inflorescence. Flowers are borne in basipetal order; in this case, which means that the older flowers are at top and younger flowers are at the base.
The flower is a reproductive part of an agiospermic plant. The flower serves the purpose of sexual reproduction. In a typical flower, there are four kinds of whorls. These whorls are successively arranged on the swollen end of the stalk or pedicel. The swollen end of the stalk is called thalamus or receptacle.
Whorls of Flower:
- Calyx: The outermost whorl of a flower is called calyx. It is composed of sepals. Sepals are usually green and leaf-like structures. The sepals protect the flower during the bud stage. Calyx is called gamosepalous when sepals are united and is called polysepalous when sepals are free.
- Corolla: The second whorl of a flower is called corolla. It is composed of petals. Petals are usually brightly coloured. The bright colours attract the insects and birds for pollination. Corolla is called gamopetalous when petals are united and is called polypetalous when petals are free. Corolla can be tubular, bell-shaped, funnel-shaped or wheel-shaped.
- Androecium: The third whorl of a flower is called androecium. It is composed of stamens. A stamen is composed of a stalk and an anther. An anther is usually a bilobed structure. There are two chambers (pollen sacs) in each lobe of an anther. Pollen sacs produce pollen grains. A sterile stamen is called staminode.
- Gynoecium: The central whorl of a flower is called gynoecium. It is composed of one or more carpels. A carpel is composed of three parts. The basal swollen portion is called ovary. The long tubular part over ovary is called style and the flat top at the style is called stigma. The style is the receptive surface for pollen grains. Each ovary bears one or more ovules which are attached to a flattened, cushion-like placenta.
Both androecium and gynoecium are present in a bisexual flower, but either of them is present in a unisexual flower.
Symmetry of flowers
- Actinomorphic: When a flower shows radial symmetry, it is called an actinomorphic flower, e.g. mustard, datura, chilli, etc.
- Zygomorhphic: When a flower shows bilateral symmetry, it is called a zygomorphic flower, e.g. pea, gulmohar, bean, Cassia, etc.
- Asymmetric: When a flower cannot be divided into two equal halves from any plane, it is called asymmetric flower, e.g. canna.
Floral parts may be present in multiples of 3, 4 or 5 in a flower. Depending on this, a flower can be trimerous, tetramerous or pentamerous.
Bracteate Flower: The reduced leaf at the base of a flower is called a bract. A flower with bract is called bracteate flower.
Ebracteate Flower: A flower without a bract is called an ebracteate flower.