Inflorescence

Angiosperms (Gr. Angion = vessel; Sperma = seed) are flowering, fruit-bearing phanerogamic, spermatophytic, and sporophytic plants. They are the most recent, most advanced, most evolved, most conspicuous, and most abundant of all the plants on this earth. The study of flowering plants is called Anthology. These plants appeared in the lower cretaceous period of the Mesozoic era about 130 million years back but flourished in the Tertiary period of the Cenozoic era. The first flowering plants appeared in the Jurassic period and monocots appeared in the Oligocene period. The first angiosperm was Archaefructus from the mid-Cretaceous. They lack archegonium which is replaced by pistil (gynaecium). They have double fertilization and endosperm of triploid nature. The male gametes are nonmotile and carried by pollen tube (siphonogamy) to avoid dependence on water for fertilization. They comprise about 12500 genera and 2.68 lakh species out of which 2.20 Lakh are dicots and 50000 are monocots. Thus dicots are more than 50% of the total plants on this earth. They show great diversity in size, form, habit, habitats, life span, and mode of nutrition. They are classified into dicotyledons and monocotyledons on the basis of the number of cotyledons. Monocots are more advanced than dicots.

What is Inflorescence?

The inflorescence is defined in different ways.

• Inflorescence (L. inflorescence to begin to blossom) is the mode of arrangement and distribution of flowers on a specialized branch called a peduncle (inflorescence axis, mother axis). [A flattened peduncle is called a receptacle.]
• It is a system of branches bearing flowers.
• It is a branch or axis bearing flowers in a definite manner.

General Characteristics of an Inflorescence

Bracts

Any leaf linked with an inflorescence is called a bract. These are modified foliage distinct from the vegetative part of the plant. It is usually located at the node from where the whole inflorescence arises and is also connected to the rachis. However, bracts can exist elsewhere in an inflorescence too. Their functions include attracting pollinators and protecting young flowers. Based on the presence or absence of bracts and their characteristics inflorescence can be of the following types:

• Bracteate Inflorescence: This type of inflorescence possesses flowers that bear small reduced leaves called bracts at their base.
• Ebracteate Inflorescence: Here the flowers don’t bear bracts.
• Leafy inflorescences: In this type of inflorescence, bracts are often reduced in size and are unspecialized. However, they look like the typical leaves of the plant.
• Leafy-bracted inflorescences: This type of inflorescence is intermediate between bracteate and leafy inflorescence.

Terminal Flowers

Plant organs grow in two different ways namely monopodial/racemose and sympodial/cymose. These two types of inflorescence are different based on whether they possess a terminal flower and the position where flowering starts within the inflorescence. Terminal flowers are flowers that are found at the extremities of the stem. The two major types of inflorescence are as follows:

• Indeterminate/Racemose Inflorescence: Here the flowers are arranged in an acropetal succession which means that new flowers are added on the top and the old flowers are found at the bottom. The main axis continues to grow as there is no flower present at its tip.
• Determinate/Cymose inflorescence: Here the flowers are arranged in a basipetal succession which means that old flowers are present above the new flowers. The main axis terminates into a flower and hence does not freely grow.

Phyllotaxis

Phyllotaxy is known as the pattern of arrangement of leaves on the stem or branch. Its types are:

• Alternate: In this type, a single leaf arises at each node in an alternate manner. Eg: China rose, Mustard, Sunflower.
• Opposite: In this type of phyllotaxy, a pair of leaves arise at each node. Example: Calotropis, Guava.
• Whorled: In this type of phyllotaxy, more than two leaves arise at a node. Example: Alstonia.

Similar to leaves, the flowers can be arranged in various ways on the stem, this is known as phyllotaxis. Similarly, the arrangement of leaves in a bud is known as Ptyxis. Other plants have bracts that subtend the pedicel or peduncle of single flowers. If a bract is attached to the stem and holding the flower (the pedicel or peduncle), it is known as recaulescent whereas when the formation of the bud is shifted up the stem distinctly above the subtending leaf, it is known as concaulescent.

Solitary Flowers

They are those flowers that are not grouped into inflorescence but occur singly in two ways.

• Solitary Terminal: Single terminal flowers develop at the tip of the main stem or its branches e.g. Poppy (Papaver).
• Solitary Axillary: The flower occurs singly in the axil of a leaf (e.g., Petunia, China Rose/Shoe Flower Hibiscus rosa-Sinensis).

Types of Inflorescence

On the basis of position, inflorescence can be terminal (e.g., Poppy), intercalary (e.g., bottle brush), and axillary (e.g., Shoe flower). Basically, the inflorescence is of the following types:

Simple Inflorescence

The peduncle is unbranched. It is of two types:

 

Racemose Inflorescence

In Racemose (indeterminate or indefinite) inflorescence, the peduncle grows continuously by apical bud and produces an unlimited number of flowers acropetally (young and smaller towards the growing point, older and larger towards the base). In case the peduncle remains small and flattened the flowers come to have a centripetal arrangement (younger towards the center, older towards the periphery). The racemose inflorescence is called simple if the peduncle is unbranched and compound if the peduncle is branched. It is of the following two types:

• Simple Racemose Inflorescence. The peduncle is unbranched. The flowers open in a centripetal manner, i.e., inner (apical) flowers open last, and outer (basal) flowers open first.
  • Typical Raceme: (Raceme) peduncle elongated unbranched, monopodial bearing cellate flowers in an acropetal manner, e.g., Delphinium (Larkspur). Raphanus (Radish). Lupinus.
  • Spike: flowers are sessile and borne acropetally on the elongated peduncles, e.g., Callistemon (Bottle Brush), Amaranthus, and Achyranthes. This is the most common type of inflorescence.
  • Spikelet: It is a compact spike having a few (1-5) flowers borne on an axis called rachilla and surrounded by two scales (= bracts) called glumes, e.g. Wheat, Oat, Grass.
  • Catkin: (Amentum) Compact pendent unisexual spike in which peduncle is thin and weak, e.g., Morous (Mulberry), Salix (Willow). Populus (Poplar), Betula (Birch). Acalypha (Red hot cat tail). This inflorescence is found in a group of families called Amentiferae.
  • Spadix: It is a modification of catkin/spike in which the peduncle is thick and fleshy with the upper part sterile (called appendix) and the lower part bearing male, neuter and female unisexual flower surrounded by a large bract called a spathe, e.g., Aroids (Colocasia), Arum, Arisaema (Cobra = Snake Plant).
  • Corymb: All the acropetally arranged pedicellate flowers come to lie at the same level due to slight shortening of the peduncle in the upper region and slight elongation of pedicels of lower flowers i.e., pedicels of flowers are of unequal length, e.g. Iberis Amara (Candytuft). Caesalpinia.
  • Corymbose-raceme: It is like a corymb near the growing point and raceme lower down, e.g., Brassica campestris (Mustard).
  • Umbel: All pedicellate flowers arise centripetally around an extremely reduced peduncle (Peduncle is reduced to a point) like the ribs of an umbrella; an involucre (whorl of bracts) present at the base of flowers. Pedicels of all flowers are of equal length, e.g., Centella (= Hydrocotyle) Asiatica (Brahmi Booti),
  • Strobile: It is a spike having persistent and membranous bracts, Humulus (Hop),
  • Capitulum: (Racemose Head, Anthodium) It is a characteristic of Compositae (Asteraceae). The peduncle is flattened and called a receptacle that bears centripetally arranged small sessile flowers called florets surrounded by an involucre of bracts, e.g., Zinnia, Marigold, Helianthus, (Tagetes), Chrysanthemum, Sonchus, Ageratum. Florets may be tubular, (= disc florets) or ligulate, (= ray florets). Capitula may be homogamous (all florets of one type), e.g., only ligulate in Sonchus and only tubular in Ageratum, or heterogamous (with two types of florets, e.g., Sunflower (Helianthus annuus) is heterogamous with both ray florets (towards periphery) and disc florets (towards the center).
  • Capitate or Spikate Head: A number of sessile flowers grow on a suppressed peduncle in a centrifugal manner forming a globose inflorescence, e.g., Acacia, Albizia, and Mimosa.
• Compound Racemose Inflorescence: The peduncle is branched, each branch bearing flowers in any racemose manner.
  • Raceme of racemes: (Compound Raceme or Panicle) The peduncle is branched in an acro petal (raceme) manner and racemes are borne acropetally on each branch of the peduncle, e.g., Cassia fistula, Delonix regia, Yucca, Asparagus, Asphodelus, the male flower of Maize (tassel).
  • Corymb of corymbs: (Compound Corymb) An axis bearing a number of corymbs in a corymbose fashion, e.g., Pyrus, Cauliflower. Edible Cauliflower represents an undeveloped inflorescence.
  • Umbel of umbels: (Compound Umbel) The branched peduncle is present in an umbel manner in rays. Each ray bears pedicellate flowers in an umbel manner (secondary umbel or umbellule). It is characteristic of the family Umbelliferae (Apiaceae). Involucre (below mother umbel) and involucels below each daughter umbel called umbellule) may be present, e.g., Coriander, Fennel, Carrot, and Cumin.
  • Spike of spikes: (compound spike) e.g., Amaranthus spinosus (chaulai).
  • Spike of spikelets: e.g., Wheat, Sorghum. In Gramineae (Poaceae, grass family) peduncle is branched. Each rachilla branch bears a group of 1-5 sessile florets called spikelets. This unit of an inflorescence is not a single flower but a group of 1-5 flowers. All spikelets are covered by two empty bracts/scales called glumes. Spikelets are sessile in wheat and pedicellate in Rice. A floret has a lemma (bract), palea (bracteole), 2 lodicules (tepal), 3 to 6 stamens, and one ovary.
  • Spike of spadices: (Compound Spadix), e.g., Date Palm, Coconut, Female flower in Maize, Banana.
  • Panicle of spikelets: e.g., Rice. Spikelets are pedicellate.
  • Capitulum of capitula: (Compound capitulum), e.g., Echinops.

Cymose Inflorescence

It is also called definite or determinate inflorescence because the growing point of the peduncle is used up in the formation of a flower. Further growth of the flowering axis is continued by one or more lateral branches which also end in flowers. Flowers are terminal, the floral axis (peduncle) is sympodial, the number of flowers is limited and flowers are arranged in a basipetal (centrifugal) manner i.e., apical (upper) flowers are older than the basal (lower) flowers and flowers open centrifugally, i.e., inner flowers open first followed by outer (basal) flowers. It is of the following types.

• Uniparous or Monochasial Cyme: The flowering axis is sympodial. As the growing point ends in a flower, further growth is continued by a lateral branch which also ends in a flower. The process is repeated. It is of two types:
  • Helicoid uniparous: All the flowers are borne on the same side, e.g. Begonia, Drosera. It can be dreponium (flowers in one plane) or bostryx (flowers in different planes).
  • Scorpioid uniparous: The flowers are borne on both sides alternately on the zig-zag peduncles, e.g. Tecoma, Freesia, Heliotropium. Rhipidium is a scorpioid cyme having all the flowers in one plane (e.g., Solanum nigrum) while in cincinnus the flowers are borne in different planes.
• Biparous or Dichasial Cyme or Dichasium: Growth of the flowering axis is continued by two branches when the growing point of the parent axis is converted into a flower, e.g., Dianthus (Pink), Silene, Nyctanthes, Jasminum, Clerodendron, Bougainvillea, Teak. The arrangement of flowers is either basipetal (when the axis is elongated) or centrifugal (if the axis is short).
• Multiparous or Polychasial Cyme or Polychasium: More than two branches continue the growth of the flowering axis when the parent axis is changed into a flower, e.g., Calotropis, Hamelia, Asclepias. The arrangement of flowers is generally centrifugal.
• Cymose Head: (Glomerule) A number of centrifugally arranged sessile or subsessile flowers are borne around a globular receptacle without involucre of bracts, e.g. Anthocephalus cadamba (Kadam).
• Scapigerous Cyme Umbel: In onion (Allium cepa) a scape (a leafless peduncle/shoot arising from terminal bud) from ground level bears an umbellate cyme covered (by one or more spathes).

Mixed Inflorescence

These inflorescences have both the characters of racemose and cymose.

• Thyrsus (Thyrse). Many cymose clusters are arranged acropetally on an axis with unlimited growth, e.g., Vitis vinifera (Grape Vine and male Cannabis).
• Mixed Spadix (Spadix of Cymes). Spadices having cymose inflorescence arranged acro petal on fleshy axis, e.g., Banana.
• Panicle of Spikelets. Spikelets are pedicellate and arranged in a compound raceme, e.g. Oat, Rice.
• Corymb of Capitula, e.g., Ageratum.
• Other Types-like umbels of capitula, cyme of capitula (e.g., Vermonia), cyme of umbels (e.g., Lantana), cyme of corymbs, etc.

Special Inflorescences

These are modified simple cymose inflorescences formed due to the overcrowding of flowers.

• Hypanthodium. It is a modified spike and cyme inflorescence adapted for myrmecophily (pollination by ants). It is a fruit-like inflorescence. It has a flask or cup-shaped fleshy receptacle, a pore (ostiole) lined by scales, and short canal-bearing hair. Internally the receptacle bears male flowers towards the ostiole, female flowers towards the base, and sterile (neuter) female flowers (called gall flowers) between the two, e.g., Ficus (Peepal, Banyan, Fig). Gall flowers contain pupa/egg of pollinating insect. These flowers are arranged in cymose groups. The receptacle of this inflorescence is formed by the condensation of the rachis of three closely placed cymes. It is cauliflorous (cladanthous) and develops from a dormant bud on an old stem.
• Coenanthium. It has an open saucer-shaped receptacle bearing florets as in hypanthodium, e.g., Dorstenia.
• Verticillaster. It is a raceme of verticals (whorls of flowers) borne on a rectangular axis in the axils of opposite leaves. At each node, there are two vertices. Each whorl consists of two clusters of 3-9 flowers with each group formed of a condensed dichasial cyme ending into monochasial scorpioid cyme, e.g., Ocimum, Salvia, Leucas, Mint (Mentha), Lavender, Coleus (all belonging to family Lamiaceae (Labiatae).
• Cyathium. It is a modified cyme that looks like a flower and is a characteristic of Euphorbia. It consists of a cup-like involucre formed by a fusion of 5 bracts that enclose a single central achlamydeous (naked) pedicellate female flower surrounded by 5 groups of male flowers. In each group, male flowers are arranged centrifugally in a uniparous scorpioid manner, e.g., Euphorbia, Poinsettia, and Pedilanthus (Jew’s slipper). In Poinsettia, bracts become red. Thus the ratio of female to male flowers is 1 many or more precisely 1: 5. Each male flower is bracteate, pedicellate, and naked without sepal, petal, and carpel and is represented by a single pedicellate stamen. There is a joint between the stamen and pedicel which represents the thalamus. The female flower is pedicellate and represented by a single tricarpellary syncarpous pistil. The female flower matures earlier than the male flower (protogyny).

Importance of Inflorescence

• It makes flowers more conspicuous to pollinating agents (insects/birds) so that chances of cross-pollination are high.
• A single pollinating agent can pollinate a number of flowers in a single visit.
• Inflorescence usually occurs away from vegetative parts and thus avoids hindrances for the pollinating agencies.
• A large number of pollens are shed from a group of flowers in the inflorescence so that air pollination becomes easier.
• As more flowers are pollinated at a time, more fruits are produced.

FAQs on Inflorescence

Question 1: What do you mean by inflorescence?

Answer:

Inflorescence refers to the arrangement of flowers on the floral axis of the plant.

Question 2: Why is inflorescence important?

Answer:

It is important as it makes flowers more conspicuous to pollinating agents and it increases the chances of cross-pollination. Moreover, Inflorescence usually occurs away from vegetative parts and thus avoids hindrances for the pollinating agencies.

Question 3: Is an inflorescence a flower?

Answer:

An inflorescence is referred to as a group or cluster of flowers, as opposed to solitary flowers borne separately.

Question 4: What are the two main inflorescence types?

Answer:

The two major types of inflorescence are Racemose and Cymose.

Question 5: Is sunflower an inflorescence?

Answer:

Yes, a sunflower is an inflorescence because the individual flowers are arranged in such a way that it looks like a normal flower.