INFLORESCENCE

            The mode of arrangement of flowers in the floral axis is called inflorescence. Flower may occur singly or in clusters. It may be terminal or axillary, and may be branched in various ways. Thus on the basis of mode of branching, inflorescences are divided into many types.

            The inflorescences may primarily be classified into two main types – racemose and cymose.

            A. RACEMOSE INFLORESCENCES: In this type of inflorescence, the main axis does not terminate in a flower but continues to grow and give of flowers laterally in acropetal succession, i.e., the lower or outer flowers are older than the upper or inner ones. The various kinds of racemose inflorescences can be described under the following heads –

            1. Main Axis Elongated:

            (a) Raceme:- In this type the main axis is elongated and it bears laterally a number of flowers which are all stalked, the lower or older flowers having longer stalk than the upper or younger ones. Examples – Radish (Raphanus sativa), Mustard (Brassica compestris), Casia, etc.

            When the main axis of the raceme is branched and the lateral branches bear flowers in raceme pattern, the inflorescence is called compound raceme or panicle. Example – Cinnamomum spp., Delonis spp., etc.

            (b) Spike:- In this type the main axis elongated and the lower flowers are older, opening earlier than the upper ones, as in raceme, but the flowers are sessile, i.e., without any pedicel or stalk. Examples – Amaranthus spp., Achyranthes spp., Adhatoda spp., etc.

            When the main axis of the spike is branched and the flowers are arranged in a spike-like manner on the branches, such inflorescence is called compound spike. Example – Amaranthus spp.

            (c) Spikelets:- These are very small spikes with one or more flowers arranged in spike, raceme or panicle. The flowers may be stalked or sessile. Each spikelet bears at its base two minute scales or bracts called empty glumes or lemna. Opposite to lemna/flowering glume a small two nerved bracteole called palea is present. Spikelets are characteristic features of the family Poaceae. Examples – Bambusa spp., Tritichum aestivum, Oryza sativa, Saccharum officinarum, etc.

            (d) Catkin:- This is a spike with a long and pendulous axis which bears unisexual flowers. Examples – Mulberry (Morus indica), Cat’s tail (Acalypha sanderiana), Betula, Quercus, etc.

            (e) Spadix:- This is also a spike with a fleshy axis. The flowers are enclosed by large and often coloured bracts called spathes. Example – Clocasia spp., Musa spp.

            2. Main Axis Shortened:

            (a) Corymb:- Here the main axis is completely short, and the lower flowers have much longer pedicels than the upper ones so that all flowers on the top brought more or less to the same level. Examples – Iberis.

            (b) Umbel:- Here the main axis is shortened, and it bears at its tip a group of flowers which have pedicels of more or less equal lengths so that the flowers are seen to be spread out from a common point so the inflorescence look like an umbel. It is the characteristics of the family Apiaceae. Examples – Carrot (Daucas carota), Coriander (Coriandrum sativum), Fennel (Foeniculum vulgare), etc.

            When the main axis of the umbel is branched and the branches bearing flowers is umbel-type, the inflorescence is called compound umbel. Example – Coriander.

            3. Main Axis Flattened or Supressed:

            (a) Head or Capitulum:- Here the main axis or receptacle is suppressed becoming almost flat, and bears numerous sessile small flowers called florets. The whole structure is looking like a single flower. The inflorescence bears a cup shaped bracts called involucres bract at its base. The outer florets called ray florets are ligulate and female or neutral, while the central florets called the disc florets are tubular and are bisexual. It is characteristics of the family Asteraceae. Examples – Sunflower (Helianthus annus), Dahlia, Zinnia, Cosmos, etc.

            B. CYMOSE INFLORESCENCES: In this type of inflorescence the growth of main axis is stopped by the development of a flower at its apex. The flowers may be with stalked or sessile. Tn cymose inflorescence, the flowers develop in basipetal succession, i.e., the terminal flower is the oldest and the lateral ones are younger. The cymose inflorescence are of three types –

            (a) Uniparous or Monochasium Cyme:- In this type the main axis ends in a flower and it produces only one lateral branch at a time ending in a flower and thus the process continues.

            Uniparous cyme inflorescence may be scorpioid or helicoids. In scorpioid type the flowers are produced laterally on alternate sides giving rise to zigzag nature (e.g., Heliotropium indicum, Hyoscyamus, etc.) while in helicoids type the lateral branches are produced in one side giving a helicoids nature (e.g., Begonia, Drosera, Solanum,etc.).

            (b) Biparous or Dichasium Cyme:- In this type of inflorescence, the main axis ends in a flower and two lateral branches develop on either side. The lateral branches terminate with flowers and thus the process continues. Example – Ixora spp., Bougainvillea spp., Dianthus, etc.

            (c) Multiparous or Polychasium cyme:- In this type of inflorescence also, the main axis ends in a flower and a number of lateral branches develop in which the same process is repeated. Example – Calotrophis, Asclepias, etc.

           C. SPECIAL TYPES OF INFLORESCENCES: Besides the normal types of inflorescence, following special types of inflorescences are found in angiosperms. These are as follows –

            (a) Cyathium:- This is a special type of modified cymose inflorescence. There is a cup shaped involucre bract, often provided with nectar secreting glands. The involucres enclose a single female flower (reduced to pistil) in the centre, seated on a comparatively long stalk, and a number of male flowers (each reduced to a solitary stamen) around this, seated on short stalks. The flowers follow the centripetal  (cymose) order of development. The female flower in the centre matures first, and then the male flowers. Example – Euphorbia spp., Poinsettia, etc.

            (b) Hypanthodium:- It is a special type of modified capitulum where the receptacle forms a hollow cavity, more or less pear shaped, with a narrow apical opening guarded by scales and the flowers are borne on the inner wall of the cavity. The flowers are unisexual, the male flowers are present near the opening and the female flowers towards the base of the cavity. Example – Ficus.

            (c) Verticillaster:- This is a condensed form of cymose inflorescence with a cluster of sessile or almost sessile flowers in the axil of a leaf, forming a false whorl at the node. The first axis gives rise to two lateral branches and succeeding ones bear only one branch each on alternate sides. It is characteristic of Lamiaceae family. Examples – Ocimum, Leonurus, Saliva, Leucas, etc.

**************