Inflorescence meaning: Inflorescence is the branch or branch-system bearing flowers or “is the arrangement of flowers on the floral axis and is a branch system”.
The stalk of the inflorescence is known as peduncle. The floral axis of an inflorescence bearing flowers is called rachis and the stalk of the individual flower is known as pedicel.
Flowers having pedicels are called pedicellate and those without pedicels i.e. stalks are sessile. Sometimes the rachis instead of developing into a long axis, is condensed to form fleshy, flat or dilated structure called receptacle.
The inflorescence axis (i.e. rachis) developing from an underground stem is known as scape. The small rachis of Gramineae i.e. grass inflorescence produced beyond the flower is also known as rachilla.
Inflorescences are mainly of three types (Green, 1904):-
I. Indefinite or Racemose inflorescence, II. Definite or Cymose inflorescence and III. Mixed inflorescence.
I. INDEFINITE OR INFLORESCENCE RACEMOSE
This 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 the following types :
A. Primary axis or rachis elongated
1. Simple forms –
(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 subtend flowers 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) Panicle 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 nucifera.
B. Primary axis or rachis short
1. Simple form :—
(i) Umbel — This inflorescence 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 Mimosoideae 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 ray florets. 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.
II. DEFINITE OR INFLORESCENCE CYMOSE
Here the growth of the primary axis is checked and is thereby terminated by a flower — young flowers occur below it. In this type flowers open basipetally or centrifugally (in case of fleshy rachis).
Cymose inflorescences are of following types :
(i) Solitary terminal or Axillary flower – In this type, apical or axillary bud develops into a single flower e.g. Hibiscus rosasinensis (Malvaceae). This type is included within cymose type as further development is suppressed.
(ii) Monochasium or Uniparous cyme – In this type, the primary axis ends in a flower and one lateral branch is produced ending in a flower and the process is repeated.
There are two kinds of uniparous cyme, such as :
(a) Scorpioid cyme or Cincinnus — In this type, the primary axis with alternate suppression of lateral flowers bearing secondary axes results in a zigzag structure. The family Boraginaceae is characterised by this inflorescence. Examples — species of Heliotropium of the family Boraginaceae, Ranunculus bulbosus (Ranunculaceae) etc.
(b) Helicoid cyme or Bostryx – In this form, the primary axis with one-sided successive suppression of lateral secondary axes forms a helix. Examples – Hamerocallis flava (Liliaceae), Hamelia patens (Rubiaceae), Myosotis palustris (Boraginaceae) etc.
(iii) Dichasium or Biparous cyme – In this the primary axis terminates into central flower which opens first; at the base of the primary axis, a little distance behind the apex, two lateral branches bearing flowers arise. Each lateral branch may again give rise to flowers in the same manner.
Examples — species of Jasminum, Nyctanthes (Oleaceae) and some members of Caryophyllaceae (Dianthus chinensis).
(iv) Polychasium or Multiparous cyme – This type of inflorescence results when the primary axis terminates in a flower and produces behind the apex more than two lateral secondary axes in a whorled manner. Examples — Sambucus nigra, Viburnum sp. (Caprifoliaceae), Calotropis sp. of Asclepiadaceae etc.
SPECIAL TYPES OF CONDENSED CYMOSE INFLORESCENCE
(i) Cyathium – It is a compound cyme, characteristic of the species of Euphorbia, Pedilanthus, etc. of the family Euphorbiaceae. In this case the receptacle terminates into a single female flower consisting of tricarpellary gynoecium only, this again is surrounded by numerous male flowers each of which consists of single stamen—these male flowers are again arranged in scorpioid cyme. The whole inflorescence is surrounded by a glandular involucre containing nectar.
(ii) Verticillaster — It is a compound cyme, characteristic inflorescence of the family Labiatae. In the axils of opposite leaves at first a dichasium is produced, then each branch of the dichasium passes into a cincinnus.
The flowers are almost sessile and the axes are so short that the flowers are crowded round the stem. So a verticillaster consists of a simple biparous cyme and scorpioid cymes. Examples — Species of Leonurus, Coleus, Ocimum, Leucas, etc. of the family Labiatae.
(iii) Hypanthodium — The inflorescence hypanthodium is composed of three fused primary axes or rachis of closely placed cymes ; as a result of such fusion a cup-shaped fleshy concave receptacle almost or completely closed structure with a small opening at the apex results.
The unisexual flowers (three types, e.g. male, fertile female and sterile female) are arranged all over the inner surface of the receptacle in cymose groups. Example – Ficus cunea (Moraceae).
(iv) Coenanthium — This is found in Dorstenia sp. (Moraceae) where the receptacle is more or less flat with little upwardly curved margins ; minute flowers are embedded within the receptacle.
(v) Cymose capitate or Cymose head — When the flowers develop on a suppressed globular rachis in a cymose manner to form a capitate or head-like structure, it is called cymose capitate. Example – Anthocephalus indica (Rubiaceae).
III. MIXED INFLORESCENCE
Besides the typical racemose and cymose inflorescences, several mixed types are often found. Here the combinations of a racemose type and cymose type or of two racemose types e.g. raceme with spike, raceme with umbel etc. are noted.
In sunflower, Dahlia, etc. the individual inflorescence is racemose, but with respect to the opening of the inflorescence the top apical inflorescence opens first and the lateral ones lower down opens later like cymose.
(a) Mixed panicle – It is a compound type and the combination of racemose and cymose or irregularly arranged flowers. The primary axis bears flowers in racemose manner and secondary axes bear flowers irregularly though terminal flowers open first like cymose. Example — Ligustrum vulgare (Oleaceae).
(b) Mixed spadix — Here cymosely arranged flower-groups are racemosely arranged on the thick fleshy primary axis in acropetal manner and each flower group is subtended by a spathe, e.g. Musa paradisiaca (Musaceae).
(c) Cymose umbel — This is seen in Allium cepa (Onion, Liliaceae), Calotropis sp. (Asclepiadaceae) etc. where cluster of cymose inflorescences are developed in “umbel” type.
(d) Cymose corymb or Corymbose cyme — Here clusters of cymose inflorescences are arranged in corymbose manner e.g. Ixora sp., Oldenlandia corymbosa (Rubiaceae), Holarrhena antidysenterica (Apocynaceae) etc. There is a special type of cymose corymb known as fascicle.
Here the successive axes of a polychasial inflorescence are of equal lengths so that flowers are arranged to the same level in a corymbose manner. e.g. Garcinia of the family Guttiferae and some species of Caryophyllaceae.
(e) Thyrsus — In this form, several cymose clusters are arranged acropetally forming the shape of a double cone on a raceme type of primary axis. Example — inflorescence of Vitis vinifera (Vitaceae).