The reproductive organs of flowering plants are flowers. The flowers are produced after a period of vegetative growth. The flowers may be borne singly or in clusters. When borne singly they are said to be solitary (eg) Hibiscus rosa sinensis (shoe flower), if in clusters they form an inflorescence.
When several flowers arise in a cluster on a common axis, the structure is referred to as an inflorescence. The common axis is the inflorescence axis which is also called rachis or peduncle. Several single flowers are attached to the inflorescence axis. In case of plants possessing underground rhizomes, the rachis or peduncle arises directly from the rhizome. Such a rachis is referred to as scape. In the case of lotus, the scape gives rise to a solitary flower. In plants like onion, the scape gives rise to an inflorescence.
Based on the location, the inflorescence may be classified into 3 types.
(i) Terminal Inflorescence
(ii) Intercalary Inflorescence and
(iii) Axillary Inflorescence.
In plants like Callistemon the inflorescence is found in between the stem. This is called intercalary inflorescence.
Generally, based on the arrangement, structure and organisation of flowers on the axis, inflorescence are classified into various types. There are four major types.
In this type, the inflorescence axis shows unlimited growth. Several flowers arise in acropetal succession on the axis. The younger flowers are found at the tip and older flowers are found towards the base of the inflorescence axis. The order of opening of flowers is centripetal i.e. from the periphery towards the centre. Racemose inflorescence may be sub-divided into various types based on branching of inflorescence axis, length of the axis and presence or absence of pedicels in flowers.
Main axis elolngated
Here the inflorescence axis is very much elongated and bears pedicellate or sessile flowers. This may include several types.
This is a very simple type of inflorescence. The axis shows unlimited growth. Numerous pedicellate flowers are arranged from base to apex in acropetal succession. Each flower arises in the axil of a bract eg. Crotolaria retusa, Cleome viscosa.
Compound Raceme or Panicle
In this type, the inflorescence axis is branched. Each branch shows flowers arranged as in a simple raceme i.e. in acropetal sucession. eg. Mangifera.
This inflorescence shows an axis of unlimited growth as in raceme but the flowers are sessile and are arranged in acropetal order eg.Achyranthes and Piper longum.
The inflorescence axis is branched and each branch is referred to as spikelet. Each spikelet bears a few flowers only. The base of the inflorescence shows a pair of bracts called glumes. Each flower has a bract called lemma and a bracteole called palea eg. Oryza(Paddy).
The inflorescence axis is swollen and fleshy. Numerous sessile flowers arranged in acropetal order are embedded in the axis. The entire inflorescence is protected and covered by a large bract called spathe. The base of the axis bears female flowers, and the sterile flowers and male flowers are borne towards the top. The tip of the inflorescence axis does not bear flowers. eg. Arum, Colocasia.
The swollen and fleshy inflorescence axis is branched and bears sessile flowers. There is a thick and large boat-shaped bract calledspathe covering the inflorescence eg. Cocos.
ii. Main Axis Shortened
Here the main axis shows reduced growth and is shortened. Corymb belongs to this type.
The inflorescence axis in this type is not elongated as in raceme. The pedicels of the flowers are of unequal length. The older flowers have long pedicels and the younger flowers show short pedicels. So all
flowers appear at the same level. eg. Caesalpinia.
iii. Main Axis ending in flowers
There are two types under this-umbel and compound umbel.
The main axis may be simple or branched. But the vertical growth of the axis is suddenly stopped and a whorl of bracts arise at the tip of the inflorescence. This is called involucre of bracts from the axils of which arise flowers having pedicels of equal length. The flowers are in acropetal order and present at the same level. eg. Allium cepa (Onion).
The main axis of the umbel inflorescence produces an involucre of bracts which give rise to branches called rays from their axils. Each ray produces an involucre of bracts at its tip from the axils of which arise flowers having pedicels of equal length in acropetal order. Each such umbel is called an umbellet. eg. Daucas carota (carrot).
Main axis flattened
The main axis is flattened and assumes various shapes. On the flattened axis flowers are arranged.
There are two types under this - head or capitulum and compound head.
Head or Capitulum
The main axis of the inflorescence is flattened and functions as the thalamus. This bears numerous florets in acropetal order. The inflorescence is surrounded by an involucre of bracts which are green in colour and protect the young flowers and fruits.
The florets of the inflorescence are sessile and are of two types. 1. The tubular or disc florets and 2. The ligulate or ray florets. Based on the type of florets present, the head inflorescence may be of two types - Homogamous head and Heterogamous head.
This type shows florets of a single kind only which may be ray or disc florets eg. Vernonia shows only disc florets and Launaeashows ray florets only.
The florets present here belong to both ray and disc type. The disc florets are present in the centre of the thalamus while the ray florets radiate outwards from the margins of the thalamus. eg. Helianthus, Tridax.
In Lagasca mollis the inflorescence axis is branched and each branch bears a head inflorescence.
Inflorescence axis shows limited growth. The tip of the inflorescence stops growing after producing a flower. The lateral pair of bracts at the base of the flower give rise to lateral branches each of which ends in a flower. Similarly the lateral pair of bracts of each of these branches may also form branches. In this way flowers are formed in basipetal order i.e. from apex to base. The older flowers is at the tip and younger flower is at the base and the order of opening of flowers is centrifugal i.e. from cnetre to periphery. The flowers are few in number.
Cymose inflorescence is of various types.
The stem or the axil of leaf may show a single flower which shows a joint on the pedicel. Such flowers are referred to as terminal solitary cyme and axillary solitary cyme respectively. eg. Papaver - Terminal solitary cyme, Hibiscus - Axillary solitary cyme.
It is a group of three flowers. The inflorescence axis ends in a flower. the two lateral bracts at the base of the flower give rise to branches ending in a flower. Thus, there are three flowers in the inflorescence and the central flower is the oldest eg. Jasminum.
The tip of the inflorescence ends in a flower. From the lateral bracts of this flower a pair of branches arise, each ending in a flower. Each of the branches bears a pair of bracts and these also give rise to a pair of lateral branches each. Thus symmetrical bunches of three flowers each are formed where the central flower is the oldest. eg. Clerodendron.
The inflorescence axis terminates in a flower. Of the two lateral bracts only one bears flowers. Such a cyme is called a monochasial cyme. This is of two types - Helicoid cyme and Scorpioid cyme.
The main axis terminates in a flower. The lateral branches arising from the axile of bracts are on one side only giving rise to a helical appearance. eg. Hamelia patens.
The main axis stops growing after producing a flower.The lateral branches arising from the axil of bracts are produced alternately to the left and to the right in a zig-zag manner eg. Heliotropium.
The main axis terminates in a flower. The lateral branches formed from the bract continue to branch repeatedly eg. Nerium.
In this type of inflorescence, the axis starts as a racemose inflorescence and shows branching in a cymose fashion. There are different types under this.
The main axis of the inflorescence shows a number of simple dichasial cymes arranged in a racemose manner eg. Ocimum.
A pair of dichasial cymes arise from the axils of opposite flowers. Later these grow as monochasial scorpioid cymes around the stem eg. Leucas.
In Musa, several cymose clusters are arranged on the swollen inflorescence axis from base to apex. Each cymose cluster is surrounded by a large bract called spathe.
The type of inflorescence which cannot be included in racemose type or cymose type is called special type. There are several kinds of special type inflorescence.
This is found in the genus Euphorbia. The inflorescence is reduced to look like a single flower. The bracts are united to form a cup - like structure enclosing
a convex receptacle. There are a number of reduced unisexual flowers on the receptacle. There is a single female flower in the centre of the receptacle. It is naked, represented by the gynoecuim only and borne on a long stalk. Around the female flower five groups of naked male flowers are arranged in a monochasial scorpioid cymes. The male flower is represented by a single stamen arising in the axil of a bract. The top of the inflorescence shows the presence of beautiful nectaries. eg. Euphorbia cyathophora.
Here the receptacle is concave and cup shaped. The upper end has an opening called ostiole, which is protected by scales. Inside the receptacle, three types of flowers are present. Male flowers are present in the upper part, female flowers towards the base and the neutral flowers are found in the middle between the male and female flowers eg. Ficus.
Here the receptacle is fleshy and appears like a circular disc like structure. The centre of the disc contains female flowers and around these are present the male flowers eg. Dorstenia.