Racemose Inflorescence

Inflorescences may also have classified based on branching, number and arrangement of flowers, and some specialized structures.

I.   Indeterminate (racemose)

II.   Determinate (cymose)

III.   Mixed inflorescence: Inflorescence of some plants show a combination of indeterminate and determinate pattern

IV.   Special inflorescence: Inflorescence which do not confined to these patterns

Racemose

The central axis of the inflorescence (peduncle) possesses terminal bud which is capable of growing continuously and produce lateral flowers is called racemose inflorescence. Old flowers are at the base and younger flowers and buds are towards the apex. It is further divided into 3 types based on growth pattern of main axis.

1. Main axis elongated

The axis of inflorescence is elongated and contains pedicellate or sessile flowers on it. The following types are discussed under main axis elongated type.

a.        Simple raceme: The inflorescence with an unbranched main axis bears pedicellate flowers in acropetal succession. Example: Crotalaria retusa, mustard and radish.

b.       Spike:  Spike  is  an  unbranched indeterminate inflorescence with sessile flowers. Example: Achyranthes, Stachytarpheta.

c.         Spikelet: Literally it is a small spike. The Inflorescence is with branched central axis. 

Each branch is a spikelet. Sessile flowers are formed in acropetal succession on the axis. A pair of inflorescence bracts called glumes is present at the base. Each sessile flower has a lemma (bract) and a palea (bracteole). Tepals reduced to colourless scaly leaves (lodicule). Each flower has stamen and pistil only. Example: Paddy, Wheat, Barley, Sorghum.

d. Catkin: Pendulous spikes with a long and drooping axis bearing small unisexual or bisexual flowers. It is also

called ament. Example: Acalypha hispida, Prosopis juliflora, Piper nigrum.

e. Spadix: An inflorescence with a fleshy or thickened central axis that possesses many unisexual sessile flowers in acropetal succession. Usually female flowers are found towards the base and male flowers are found at the apex. Entire inflorescence is covered by a brightly coloured or hard bract called a spathe. Example: Amorphophallus, Colocasia, Phoenix, Cocos.

f. Panicle: A branched raceme is called panicle. Example: Mangifera, neem, Delonix regia. It is also called Compound raceme or raceme of racemes.

2. Main axis shortened:

Inflorescence with reduced growth of central axis. There are two types namely corymb and umbel.

a. Corymb: An inflorescence with shorter pedicellate flowers at the top and longer pedicellate flowers at the bottom. All flowers appear at the same level to form convex or flat topped racemose inflorescence. Example: Caesalpinia. Compound corymb: A branched corymb is called compound corymb. Example: Cauliflower.

b. Umbel: An inflorescence with indeterminate central axis and pedicellate flowers arise from a common point of peduncle at the apex. Example: Allium cepa, Centella asiatica, Memecylon umbellatum. Compound umbel: It is a branched umbel. Each smaller unit is called umbellule. Example: Daucas carota, Coriandrum sativum, Memecylon edule.

3. Main axis flattened:

The main axis of inflorescence is mostly flattened (convex or concave) or globose. A head or capitulum is a determinate or indeterminate, group of sessile or sub sessile flowers arising on a receptacle, often subtended by an involucre.

a. Head: A head is a characteristic inflorescence of Asteraceae and is also found in some members of Rubiaceae.

Example: Neolamarkia cadamba, Mitragyna parvifolia and in some members of Fabaceae-Mimosoideae. Example:

Acacia nilotica, Albizia lebbeck, Mimosa pudica (sensitive plant).

Torus contains two types of florets: 1. Disc floret or tubular floret. 2. Ray floret or ligulate floret.

The flower and inflorescence are subtended by a lateral appendage called bract. In sunflower, you may notice that the whorl of bracts forms a cup like structure beneath mimicking the calyx. Such whorl of bracts is called involucre. A group of bracts present beneath the sub unit of inflorescence is known as Involucel.

Heads are classified into two types.

i. Homogamous head: This type of inflorescence exhibits single kind of florets. Inflorescence has disc florets alone. Example: Vernonia, Ageratum or Ray florets alone. Example: Launaea, Sonchus.

ii. Heterogamous head: The inflorescence possesses both types of florets. Example: Helianthus, Tridax.

Disc florets at the centre of the head are tubular and bisexual whereas the ray florets found at the margin of the head which are ligulate pistilate (unisexual).