Inflorescence

Based on the mode of branching of the main axis or peduncle, the main groups of inflorescences are: 1) simple (single), 2) compound, 3) mixed, and 4) special.

1) Simple (Single) Inflorescences

The simple inflorescence is of two types: racemose inflorescence and cymose inflorescence.

a) Racemose Inflorescence

It is an indefinite or indeterminate type of inflorescence, showing indefinite growth of the main axis and bears numerous flowers due to the presence of an active growing point. Here, the flowers are arranged in one of two ways – acropetal or centripetal. In the former, the youngest flowers are towards the apex, and the oldest flowers are towards the base of the inflorescence. In the latter, the youngest ones are towards the center, and the oldest flowers are towards the periphery.

Racemose inflorescence can be divided into two types, simple and compound, depending on the development of the main axis and pedicels of the flowers.

With Long Main Axis

i) Simple Raceme: Unbranched, elongated peduncle bearing pedicellate or stalked flowers acropetally

Examples – mustard, radish, Delphinium, Larkspur, Veronica, and Linaria

ii) Corymb: Like a typical simple raceme, unbranched peduncle bears pedicellate flowers in an acropetal way. Here, the pedicels of older flowers are longer than those of younger ones so that all flowers lie at the same level.

Examples – candytuft, Lantana, and cherry

iii) Corymbose Raceme: In this case, the young flowers are arranged like a corymb, but in the mature stage, the longer pedicels of the lower flowers do not bring them to the level of upper ones.

Examples – mustard

iv) Spike: Sessile flowers borne acropetally over an elongated peduncle

Examples – Malabar nut, chaff-flower, bottle brush, Amaranthus, and Verbena.

v) Spikelet: It is a compact dry spike with only one or a few sessile flowers born on a specialized axis called the rachilla. It remains surrounded at the base by two scaled bracts called glumes.

Examples – wheat, oat, and grass

vi) Catkin: It is a compact pendulous spike with a hanging axis bearing sessile unisexual flowers that matures and falls as a single unit. Here, the peduncle is thin, long, and weak.

Examples – mulberry, willow, poplar, red hot Cattail, Betula, Quercus, birch, and oak

vii) Spadix: It is a specialized spike possessing a thick fleshy peduncle and a large green or colored bract called the spathe. Here, the peduncle bears a colored and sterile appendix at the upper part. In contrast, its lower part holds sessile uni­sexual flowers, lower female and upper male. These two types of flowers are separated by downwardly directed sterile hair or neuter flower.

Examples – Colocasia, arum, maize, banana, and palm

With Short and Flattened Main Axis

i) Simple Umbel: More or less equally heightened pedicellate flowers arranged centripetally around a significantly reduced peduncle with an involucre or a whorl of bracts below. Here, the main axis above the first flower is so shortened that the internodes between the successive flowers become suppressed.

Examples – Water hyssop, water pennywort, and Centella

ii) Capitulum (Head): The peduncle is reduced to a flat, concave, or convex disc, the receptacle that bears small sessile and centripetally arranged flowers called florets, surrounded by an involucre of bracts.

Examples – cosmos, tagetes, zinnia, chrysanthemum, and sunflower

iii) Hypanthodium: In this type, the peduncle gets modified in a narrow cup-like structure. Here, the female flowers develop at the base of the cup while the male flower develops towards the mouth.

Examples – Ficus benghalensis and Ficus religiosa

b) Cymose Inflorescence

It is a definite or determinate inflorescence where the tip of the main axis terminates into a flower. Further, it divides into one or more lateral branches, which also behave like the main axis. The stalked flowers are either basipetal or centrifugal. The terminal flower is the oldest in basipetal, and young flowers are present on the lower side. In centrifugal, the older flowers are in the middle, and the younger ones are present in the periphery.

Depending on the number of lateral branches formed, cymose inflorescence can be uniparous, biparous, or multiparous.

Uniparous Cyme

The main axis terminates in a flower below which a daughter axis is produced, ending in a flower. The process is repeated many times. Uniparous cyme is of two types:

i) Scorpioid Cyme: Successive daughter axes develop alternately on the right and left, forming a zigzag.

Example – Freesia

ii) Helicoid Cyme: Successive daughter axes are developed on the same side, either right or left, forming a helix or spiral.

Examples – Drosera and Begonia

Biparous Cyme

The peduncle terminates in flower and gives rise to two daughter axes that end in flowers. The process gets repeated. This type usually passes into a uniparous cyme by suppressing a branch at each subsequent branching.

This type of cyme is found in Ipomea, Stellaria, Dianthus, Nyctanthes, Spergula, Silene, Jasmine, Clerodendron, Bougainvillea, and Teak.

Multiparous Cyme

The main axis or peduncle terminates in a flower. It gives rise to three or more daughter axes, and each continuously branches in the same manner.

It is commonly seen in Euphorbia, Hamelia patens, Calotropis, and Asclepias.

2) Compound Inflorescences

In this type of inflorescence, the peduncle repeatedly branches once or twice in a racemose or cymose manner. For the first case, it becomes a compound raceme, and for the latter, it becomes a compound cymose inflorescence.

Its five subtypes are as follows:

a) Compound Raceme (Panicle)

It consists of several simple racemes borne in a racemose manner on the main axis.

Example – Delphinium

b) Compound Spike

The main axis of the inflorescence bears small lateral spikes instead of sessile flowers.

Examples – wheat and barley

c) Compound Umbel

Several umbels are borne on the tip of the main axis. Bracts, if present, remain at the base of the primary umbel.

Examples – coriander and carrot

d) Compound Corymb

The main axis divides in a corymbose manner, and each branch forms a simple corymb.

Example – Cauliflower

e) Compound Capitulum

The secondary capitulum consists of a single flower surrounded by an involucre (bracts present at the base of the flower).

Examples – echinus and veronica

3) Mixed Inflorescences

Mixed Inflorescence shows a combination of indeterminate and determinate patterns, i.e., racemose and cymose patterns.

It is of the following two types.

a) Thyrsus

It is a ‘Raceme of cymes.’ Here, the indefinite central axis bears lateral pedicellate cymes.

Examples – Ocimum and Anisomeles

b) Verticillaster

The main axis bears two opposite lateral sessile cymes at the node’s axil. Each of them produces monochasial scorpioid lateral branches so that flowers are crowded around the node.

Examples – Leonotis and Leucas

4) Special Inflorescences

In some inflorescences, it becomes challenging to make out the actual mode of branching, as the daughter axes get significantly reduced and remain crowded in many groups. Therefore, these are categorized as special inflorescences.

a) Cyathium

This inflorescence is highly reduced and appears like a single tiny flower. In this case, five green or colored bracts fuse to form a deep cup-like involucre enclosing reduced male flowers, a single female flower, and nectariferous glands.

It occurs in Poinsettia and Eu­phorbia species.

b) Hypanthodium

It has a flask-shaped fleshy receptacle. It also possesses a short canal lined by downwardly pointed hairs and a terminal pore at one end. Internally the receptacle bears female flowers towards the base, male flowers towards the pore, and sterile flowers between the two groups.

 Examples – Ficus benghalensis, Ficus religiosa and fig

c) Coenanthium

Saucer-shaped receptacle with upturn margin bearing florets

Example – Dorstenia

• During the blossoming period, inflorescence presents the flowers in unique ways, allowing the transfer of pollen, thus optimizing the reproductive success of the plant species.
• It connects the vegetative stages in a plant’s life cycle with the flowers, providing the context in which effective pollen transfer and fruit set occur.
• It provides nutrients to the developing flowers and fruits.
• It supports the mature fruits before dispersal and facilitates effective fruit and seed dispersal.
• From a structural viewpoint, inflorescences have played essential roles in systematic and phylogenetic studies.
• As functional units, they facilitate reproduction and are primarily shaped by natural selection.