Have you seen a bouquet being used during functions? Group of flowers arranged together on our preference is a bouquet. But an inflorescence is a group of flowers arising from a branched or unbranched axis with a definite pattern. Function of inflorescence is to display the flowers for effective pollination and facilitate seed dispersal. The grouping of flowers in one place gives a better attraction to the visiting pollinators and maximize the energy of the plant.
Have you ever noticed the inflorescence arising from different positions? Where is the inflorescence present in a plant? Apex or axil?
Based on position of inflorescences, it may be classified into three major types. They are,
Terminal : Inflorescence grows as a part of the terminal shoot. Example: Raceme of Nerium oleander
Axillary: Inflorescence present in the axile of the nearest vegetative leaf. Example: Hibiscus rosa-sinensis
Cauliflorous : Inflorescence developed directly from a woody trunk. Example: Theobroma cocoa, Couroupita guianensis
Observe the inflorescence of Jackfruit and Canon ball tree. Where does it arise?
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
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.
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.
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.
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).
Central axis stops growing and ends in a flower, further growth is by means of axillary buds. Old flowers present at apex and young flowers at base
1. Simple cyme (solitary): Determinate inflorescence consists of a single flower. It may be terminal or axillary. Example: terminal in Trillium grandiflorum and axillary in Hibiscus.
Cyme (uniparous): The main
axis ends with a flower. From two lateral bracts, only one branch grows
further. It may be helicoid (bostryx)
or Scorpioid (cincinnus).
a. Helicoid: Axis develops on only one side and forms a coil structure atleast at the earlier development stage. Example: Hamelia, potato.
b. Scorpioid: Axis develops on alternate sides and often becomes a coil structure. Example: Heliotropium.
3. Simple dichasium (Biparous): A central axis ends in a terminal flower; further growth is produced by two lateral buds. Each cymose unit consists of three flowers of which central one is old one. This is true cyme. Example: Jasminum.
4. Compound dichasium: It has many flowers. A terminal old flower develops lateral simple dichasial cymes on both sides. Each compound dichasium consists of seven flowers. Example: Clerodendron.
A small,simple dichasium is called cymule
5. Polychasial Cyme (multiparous): The central axis ends with a flower. The lateral axes branches repeatedly. Example: Nerium
Sympdial Cyme: In monochasial cyme, successive axes at first develop in a zigzag manner and later it develops into a straight pseudo axis.
Example: Solanum americanum.
Inflorescences in which both
racemose and cymose patterns of development occur in a mixed manner. It is of
the following two types.
1. Thyrsus: It is a ‘Raceme of cymes’. Indefinite central axis bears lateral pedicellate cymes, (simple or compound dichasia). Example: Ocimum, Anisomeles.
2. Verticil or Verticillaster: Main axis bears two opposite lateral sessile cymes at the axil of the node,each of it produces monochasial scorpioid lateral branches so that flowers are crowded around the node. Example: Leonotis, Leucas.
The inflorescences do not show any of the development pattern types are classified under special type of inflorescence.
1. Cyathium: Cyathium inflorescence consists of small unisexual flowers enclosed by a common involucre which mimics a single flower. Male flowers are organised in a scorpioid manner. Female flower is solitary and centrally located on a long pedicel. Male flower is represented only by stamens and female flower is represented only by pistil. Cyathium may be actinomorphic (Example: Euphorbia) or zygomorphic (Example: Pedilanthus.). Nectar is present in involucre.
2. Hypanthodium: Receptacle is a hollow, globose structure consisting unisexual flowers present on the inner wall of the receptacle. Receptacle is closed except a small opening called ostiole which is covered by a series of bracts. Male flowers are present nearer to the ostiole, female and neutral flowers are found in a mixed manner from middle below. Example: Ficus sp. (Banyan and Pipal).
3. Coenanthium: Circular disc like fleshy open receptacle that bears pistillate flowers at the center and staminate flowers at the periphery. Example: Dorstenia