The arrangement of flowers on floral axis or peduncle resulting in formation of groups is called inflorescence.
An inflorescence may be; simple, compound or of special types according to the mode of branching.
A simple inflorescence maybe racemose or cymose according to the mode of branching.
In this type of inflorescence, the main axis continues to grow until last flower is formed at the apex. The younger flowers present near the apex and older near the base (acropetal succession). In some cases, the main axis is compressed or flattened into a disc bearing a cluster of flowers on its upper surface. In these cases, the oldest flowers are at periphery and younger ones in the center (centripetal arrangement).
The common type of racemose inflorescence met among different flowering plants are:
Simple Raceme: The main axis elongated, flowers pedicellate, the pedicels of flowers are of same size, e.g., in Delphinium ajacis (Ranunculaceae).
Corymb: The main axis elongated, flowers pedicellate, the pedicels of older flowers longer than those of younger flowers, so all the flowers are arranged at same level, e.g., in Iberis amara (cadytuft).
Corymbose Raceme: The inflorescence is corymbose in flowering stage but later becomes a simple raceme due to elongation of axis, e.g., in Brassica (Brassicaceae).
Spike: The main axis elongated bearing sessile flowers, e.g., in Bottle-brush.
Catkin: A pendulous spike bearing unisexual flowers as in Mulberry.
Spadix: A thick fleshy spike, may be unbranched, enclosed in one or more bracts, e.g., in Maize, Banana, etc.
Simple Umbel: The growth of internodes suppressed after formation of first flower. The flowers are pedicellate and arranged at same level. The older flowers are towards periphery and younger ones in the center, e.g., in cherry.
Capitulum: The floral disc is reduced to a flattened disc, the receptacle; and cluster of small flowers, the florets are borne on the disc, therefore the inflorescence appears to be a single flower. The receptacle is surrounded by overlapping bracts that form a cup-shaped involucre at the vase of the receptacle. If all the florets in a capitulum are of same kind, it is homogamous as in Sonchus. However, in sunflower (Asteraceae) the florets are of two types:
- Ray Florets – Ligulate, bisexual or neuter and arranged along the periphery of the disc.
- Disc Florets – Tubular, bisexual and arranged in the center of the receptacle. This type of capitulum is called heterogamous.
Hypanthodium: In this case the receptacle grows upwards along its margins to form a pear-shaped structure, with an apical opening guarded by scales. Unisexual flowers develop on the inner surface of the hollow receptacle. The male flowers are present near the apex while the female flowers occupy the rest of the surface. This type of inflorescence is characteristics of Banyan, Fig, Peepal, etc.
In this type of inflorescence, the main floral axis ends in a flower soon; and one, two or more lateral branches develop below the terminal flower. Each daughter axes ends in a flower and produces lateral branches. The flowers develop in basipetal succession, i.e., older flowers present towards periphery and younger towards base. The arrangement of flowers in cymose inflorescence is centrifugal, i.e., the older flowers occupy the center and younger periphery of the cluster.
The cymose inflorescence may be:
Uniparous Cyme: When a single daughter axes is produced below the terminal flower. It may be: scorpoid cyme, when the successive daughter axes develop on right and left side alternately forming a zigzag, e.g., in Ranunculus (Ranunculaceae); or helicoid cyme, where the successive daughter axes develops on the same side, either right or left, e.g., in Begonia. In both scorpoid and helicoid cymes the basal parts of the successive daughter axes become straightened up to form a false central axis or sympodium, so that the flowers come to lie opposite the bracts.
Biparous Cyme: In this case, the main axis ends in a flower and produces two daughter axes, e.g., in Stellaria media, Ipomea, etc.
Multiparous Cyme: The main axis produces more than two daughter axes below the terminal flower, e.g., in Calotropis, Euphorbia, it looks like an umbel but can be distinguished by the opening of the middle flower first.
In compound inflorescence, the main axis is branched and bear flowers in the same manner as on the main axis. The most common types of compound inflorescence are:
Compound Raceme: It consists of several racemes borne in a racemose manner on the main axis, e.g., in Delphinium ajacis.
Compound Corymb: In this case, the main axis is branched and bears a number of corymbose, e.g., in cauliflower (phool ghobi).
Compound umbel: The inflorescence comprises of a number of umbels borne in umbellate manner at the tip of main axis.
Compound Spike: The main axis bears a number of sessile lateral spikes called spikelets – each comprising of one to few flowers and enclosed in specialized bracts, e.g., in Avena sativa (barley), Triticum (wheat).
In some inflorescence, it becomes difficult to make out the real mode of branching because the daughter axes may be abbreviated or suppressed or the flowers are crowded in many groups. Such inflorescence is called special inflorescence. These may be:
Cyathium: The main axis ends in a reduced female flower that is tricarpellary and syncarpous pistil borne at the end of a long pedicel. Five scorpoid cymose clusters of reduced male flowers surround the stalk of female flower. Each male consists of a single stamen and a scaly bract at its base. The perianth leaves are completely suppressed and flowers are enclosed in a cup-shaped involucre. Cyathium is characteristic inflorescence of Euphorbias.
Verticillaster: It is a condensed cymose inflorescence. The clusters of flowers are borne in the axis of opposite decussate bracts. The two clusters of flowers form a whorl at the node called veerticillaster. The cluster of the flower exhibit bigamous cymose arrangement first but later becomes uniparous cymose due to the suppression of the growth of daughter axis. The verticillaster is found in Ocimum (niazbo), Salvia, Mint. Etc.
Scapigerous or Umbellate Cymose Head: In onion, the flowers are borne at the end of long leafless stalk, the scape arises in the midst of the radicle leaves. The flowers are arranged in many helicoid cymose clusters. The inflorescence appears like an umbel, they are also called umbellate cymose head.
Cymose Head: In Acacia (kikar) and Albizzia (shirin) the flowers are arranged in clusters forming a head. Actually, each cluster is a scorpoid cyme in which the main axis and successive daughter axes have been reduced and the flowers are almost sessile.