The modified stem that bears flowers and helps in reproduction is called `peduncle'. It produces flowers in a definite manner. The mode of development and arrangement of flowers on the peduncle is called 'inflorescence'. The flowers are attached to the peduncle with their stalks. These stalks are called 'pedicels'. Flowers with pedicels are called 'pedicellate'. In some flowers the pedicel is absent and they are called `sessile'. These flowers are attached directly to the peduncle.
Based on their origin the inflorescences are of three types:
- Terminal Inflorescence
These inflorescences are developed from the apices of main stem or branches e.g. Crotalaria and Croton.
- Axillary Inflorescence
They are borne in the axils of leaves e.g. Dolichos.
- Intercalary Inflorescence
This type of inflorescence is borne at the nodes. After the formation of inflorescence, again the stem continues its growth e.g. Callistemon.
- Racemose or indefinite
- Cymose or definite
- Mixed type
- Special type
Racemose or Indefinite Inflorescence
In this type the peduncle grows indefinitely and produces a number of flowers directly or indirectly on its branches in acropetal manner.
Based on the presence and absence of pedicels the racemose inflorescences are mainly of two types.
- Pedicellate Racemose inflorescence and
- Sessile Racemose inflorescence
- Pedicellate Racemose Inflorescence
These are racemose inflorescence with pedicellate flowers. They are of the following types:
- Simple Raceme
The peduncle is simple, unbranched, elongated, producing many pedicellate, bracteate flowers in acropetal succession e.g. Crotalaria.
- Compound Raceme
It is also called `Panicle'. In this, the peduncle is branched and each branch produces pedicellate, bracteate flowers acropetally e.g. Mangifera.
- Simple Corymb
The peduncle grows indefinitely and actively producing many pedicellate flowers in acropetal manner. The lower flowers have longer pedicels and apical flowers have shorter pedicels. Thus all the flowers are brought more or less to the same height e.g. Gynandropis.
- Simple Umbel
The peduncle is condensed and unbranched. Many pedicellate and bracteate flowers arise from its apex in a cluster. All the flowers open in a centripetal manner. At the base of flowers, all the bracts are united to form a whorl called `involucre' e.g. Allium cepa.
- Simple Raceme
- Sessile Racemose Inflorescence:
These are seven types:
- Simple Spike
The peduncle is racemose, unbranched and produces bracteate, sessile flowers acropetally. e.g. Achyranthus.
- Compound Spike
In this inflorescence the peduncle is branched and grows indefinitely. The branches produce bracteate, sessile flowers acropetally. These branches are called `spikelets'. e.g. Oryza.
It is a spike-like inflorescence, but the peduncle is long, weak and drooping, unisexual flowers acropetally. e.g. Casuarina.
- Simple Spadix
In this, the peduncle is fleshy, unbranched and produces many sessile bracteate, unisexual flowers acropetally. Female flowers are at the base and male flowers at the apex. Normally the peduncle remains flowerless at the tip. One of the bract is modified into a thick, leathery structure called `spathe'. It covers the entire inflorescence e.g. Colocasia.
- Head or capitulum
In this inflorescence the axis is condensed into a flattened disc. Many small sessile flowers (florets) are closely arranged on this receptacle. The entire inflorescence is covered and protected by involucre of bracts. The head inflorescence is of two types, homogamous head and heterogamous head.
- Simple Spike
Fig: Types of Racemose Inflorescence (a) Raceme (b) Spike (c) Spikelet (d) Catkin
Cymose or Definite Inflorescence
In this type of inflorescence the growth of the peduncle is stopped due to the development of a flower at its apex. The bracts on the axis below this flower produce branches. These branches also terminate into a single flower. On the elongated axis the inflorescence bears the first formed flowers at the apex and younger flowers at the base. This type of arrangement is called `basipetal arrangement'.
The cymose inflorescences are as follows:
- Solitary Cyme
The inflorescence axis is branchless and bears a single flower at its apex. (e.g. Datura)
- Simple Cyme or Cymule
In this the inflorescence axis terminates into a flower. Below this flower the main axis produces two branches each of which also terminate into a flower. Thus a three-flowered inflorescence is formed. (e.g. Jasminum)
- Monochasial Cyme
It is a type of cymose inflorescence in which the inflorescence axis terminates into a flower and produces only one branch from its basal branch. This branch also grows definitely and ends with a flower. In this way many single branches develop to form a psudoaxis. This axis is also called 'sympodial axis' (e.g. Solanum).
- Dichasial Cyme
In this type, after terminating into a flower the inflorescence axis produces two lateral branches below it (e.g. Clerodendron).
- Polychasial Cyme
In this type, the inflorescence axis terminates into a flower and produces many lateral branches from its base (e.g. Nerium).
It is a type of inflorescence, which shows both the characteristics of racemose and cymose types (e.g. Thyrsus).
Special Types of Inflorescences
Due to modification in some inflorescences the arrangement and opening of the flowers remain special. They are of the following types:
The inflorescence axis consists of many nodes and each node contains two opposite bracts. Both the inflorescences of opposite bracts untie around the node like a false whorl. Hence it is called as `verticellaster' (e.g. Leucas).
It is a single flower-like special inflorescence found in Euphorbiaceae members. The inflorescence is covered by a deep cup-like involucre of bracts with external nectaries. At the centre of this cup there is a single female flower and many male flowers are arranged around it (e.g. Euphorbia).
It is a fruit-like inflorescence. The inflorescence axis is condensed and forms a fleshy, cup-like structure with an apical opening. Small, sessile, unisexual flowers develop on the inner wall of the cup. Male flowers are near the apical opening and female flowers at the base (e.g. Ficus).
Special Types of Inflorescence (a, b) L.S. of Cyathium (c) Verticillaster (d) L.S. of Hypanthodium