Indefinite or Racemose Inflorescence and its types- digieduco

Indefinite or Racemose Inflorescence


Racemose type is also known as botryose. In this type, the primary floral axis i.e. rachis is never terminated by a flower, but continues to grow giving off flowers laterally in acropetal order i.e oldest flower near the base and the youngest towards the apex, or flowers opening in centripetal manner on a fleshy and dilated rachis called receptacle.

Indefinite or racemose inflorescence consists of following types:
A. Primary axis or rachis elongated:-
(i)Raceme- primary axis or rachis long , bearing pedicellate i.e stalked flowers
opening from base to apex i.e. flowers open in acropetal order. Examples-Cleome viscosa (Capparidaceae),'Crotalaria juncea (Papilionaceae), Brassica juncea, B. nigra (Brassicaceae), Croton bonplandianum (Euphorbiaceae) etc. ; found also in Other dicotyledons as well as monocotyledons.

This type is not helpful in determining a particular family as it occurs in many families.

(ii) Corymb-A typical raceme with unequal flower stalks i.e. pedicels , so that the basal flowers stand on the same level as upper ones e.g. Prunus cerasus (Rosaceae) and Cassia sp. (Caesalpinaceae).

(iii) Spike-Like raceme but flowers are sessile e. g. Aerva aspera, Deringia amaranthoides (Amaranthaceae) and others. It occurs in numerous families. No particular family can be assigned to it.

(iv) Amentum or Catkin-It is a spike of unisexual flowers with weak drooping peduncle which ultimately breaks off from the parent plant. Examples-families Juglandaceae (Engelhardtia spicata), Betulaceae (Corylus colurna), Salicaceae (Salix tetrasperma) etc. are characterised by this inflorescence. Catkin is also found in some members of Euphorbiaceae, e.g.Acalypha hispida, Aporosa sp. etc.

(v) Spadix-It is a modified spike with fleshy rachis and large bract (the spathe) covering the inflorescence. It is found in members of Araceae (species of Colocasia, Amorphophallus etc.) as well as in many species of Palmae.

(vi) Locusta or Spikelet-This is a condensed spike subtending one or few flowers; the rachilla i.e. floral axis may be very small or long produced beyond the glumes-the
so called bracts of this inflorescence. In a spikelet the lower first and second bracts, known as glumes, are empty i.e. without flowers ; other bracts above it subtendflowers in their axils known as flowering glumes or lemmas. Opposite to each lemma there stands a bracteole called palea. In a flower the perianth is usually represented by 2 or 3 lodicules which are very small, membranous and cuneate structures at the base of the ovary. This type of inflorescence is the characteristic of the families Gramineae and Cyperaceae. But the ultimate inflorescence formed by the spikelets are complex and compound. In Oryza sativa (rice) the spikelets are one flowered as described above and the entire inflorescence is branched like a panicle. In Triticum aestivum (wheat) the spikelets are many flowered and are borne on unbranched axis looking like a compound spike.

(vii) Strobile-It is a modified spike, consisting of persistent membranous bracts or scales bearing pistillate (female) flower at each axil. Example-Humulus lupulus (the hop, Cannabinaceae).

2. Compound forms :
Modification of simple forms-here the primary floral axis or rachis bears secondary branches upon which flowers are developed.

Following are the main types-
(i) Particle or Compound Raceme-Here the flowers are borne on the branches of the main axis (i.e., branched secondary axis) instead of on the primary axis directly. This type is common in flowering plants e.g., Mangifera indica (Anacardiaceae), Vatica sp. (Dipterocarpaceae), Litchi chinensis (Sapindaceae) etc.
(ii) Compound corymb, also known as panicled corymb. Here flowers are borne in corymbose manner on the branches of the main axis or rachis. Examples-Pyrus communis (Rosaceae), Cassia siamea (Caesalpinaceae) etc.

(iii) Compound spike-When sessile flowers arise on branches of the main axis in a Spikate manner. Examples-Amaranthus spinosus, A. viridis, etc. Family Amaranthaceae is Characterised by this type.
(iv) Compound spadix-This type is noted in Arecaceae i.e.Palmae where the axis Of a spadix branches. The entire structure is covered by a stiff boat-shaped spathe.Example-Cocos nuczfera.

B. Primary axis or rachis short
1. Simple form :

(i) Umbel-This infiorescence has a short floral axis upon which pedicellate flowers are arranged in radiating manner, the order of development of flowers is centripetal. The family Umbelliferae (Apiaceae) is characterised by this inflorescence. Common example is Centella asiatica.

2. Compound form :
(i) Compound umbel-also characteristic of the family Apiaceae. In this type the axis of an umbel branches, as a result compound umbel is formed. Example species of Foeniculum, Coriandrum, etc.

(ii) Capitate-Here several sessile flowers grow from a suppressed rachis, as a result more or less globose structure is formed-this is known as capitate inflorescence. Example-Many plants belonging to sub-family Mimosoideaé of Leguminosae such as Albizzia sp. Acacia, Mimosa, etc. Like capitulum there is no receptacle.

C. Primary axis fleshy and dilated
(i) Capitulum, Head or Anthodium-Here the rachis is called receptacle, it is fleshy . and dilated ; the flowers are sessile and small, sometimes the marginal flowers are quite different from the central ones. The marginal flowers are ligulate, female or Sterile, and known as rayflorets. The centrally situated flowers are bisexual and known as disc florets. Individual florets are usually subtended by scaly bracteoles. The whole inflorescence is sheathed by involucral bracts ; the opening of flowers is centripetal. This inflorescence is the characteristic of the family Compositae, e.g. species of Helianthus, Tridax, etc.