inflorescence

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An inflorescence is a group or cluster of flowers arranged on a stem that can have only one main branch or be composed of complicated arrangements of branches. Speaking more strictly, it is the part of the shoot of the spermatophytes where flowers are formed and which is accordingly modified. The modifications can involve the length and the nature of the internodes and the phyllotaxis, as well as variations in the proportions, compressions, swellings, adnations, connations and reduction of main and secondary axes.

The stem holding the whole inflorescence is called a peduncle and the main stem holding the flowers or more branches within the inflorescence is called the rachis. The stalk of each single flower is called a pedicel.

A flower that is not part of an inflorescence is called a solitary flower and its stalk is a peduncle.

The fruiting stage of an inflorescence is known as an infructescence.

General characteristics

Inflorescences are described by many different characteristics including how the flowers are arranged on the peduncle, the blooming order of the flowers and how different clusters of flowers are grouped within the inflorescence. These terms are often idealised representations and plants in nature can have combinations of different types.

Bracts

Inflorescences usually have modified foliage different from the vegetative part of the plant. Considering the broadest meaning of the term, any leaf associated with an inflorescence is called a bract . A bract is usually located at the node were the main stem of the inflorescences forms, joined to the main stem of the plant, but other bracts can exist with in the inflorescence itself. According to the presence or absence of bracts and their characteristics we can distinguish

• Ebracteate inflorescences: No bracts in the inflorescence.
• Bracteate inflorescences: The bracts in the inflorescence are very specialised, sometimes reduced to small scales, divided or dissected.
• Leafy inflorescences: Though often reduced in size, the bracts are unspecialised and look like the typical leaves of the plant, so that the term flowering stem is usually applied instead of inflorescence. It should be noted that this use is not technically correct, as, despite their 'normal' appearance, these leaves are considered, in fact, bracts, so that 'leafy inflorescence' is preferable.
• Leafy-bracted inflorescences: Intermediate between bracteate and leafy inflorescence.

If many bracts are present and they are strictly connected to the stem, like in the family Asteraceae, the bracts might collectively be called an involucre. If the inflorescence has a second unit of bracts further up the stem, they might be called an involucel.

Ebracteate inflorescence.	Ebracteate of Wisteria sinensis	Bracteate inflorescence.	Bracteate inflorescence of Pedicularis verticillata.
Leafy-bracted inflorescence.	Leafy-bracted inflorescence of Rhinanthus angustifolius.	Leafy inflorescence.	Leafy inflorescence of Aristolochia clematitis.

Terminal flower

Plant organs can grow according to two different schemes, namely monopodial and sympodial. In inflorescences these two different growth patterns are called indeterminate or determinate, and indicate whether a terminal flower is formed and where flowering starts within the inflorescence.

• Indeterminate inflorescence: Monopodial growth. The terminal bud keeps growing and forming lateral flowers. A terminal flower is never formed.
• Determinate inflorescence: Sympodial growth. The terminal bud forms a terminal flower and then dies out. Other flowers then grow from lateral buds.

Indeterminate and determinate inflorescences are sometimes referred to as open and closed inflorescences respectively.

In determinate inflorescences the terminal flower is usually the first to mature (praecursive development), while the others tend to mature starting from the bottom of the stem. This pattern is called acropetal maturation. When flowers start to mature from the top of the stem , maturation is basipetal, while when the central mature first, divergent.

Determinate inflorescence with acropetal maturation

Determinate inflorescence with basipetal maturation

Determinate inflorescence with divergent maturation

In indeterminate inflorescence there is no true terminal flower and the stem usually has a rudimentary end. In many cases the last true flower formed by the terminal bud (subterminal flower) straightens up, appearing to be a terminal flower. Often a vestige of the terminal bud may be noticed higher on the stem.

Indeterminate inflorescence with a perfect acropetal maturation.

Indeterminate inflorescence with a acropetal maturation and lateral flower buds.

Indeterminate inflorescence with the subterminal flower to simulate the terminal one (vestige present)

Phyllotaxis

As with leaves, flowers can be arranged on the stem according to many different patterns. See 'Phyllotaxis' for in-depth descriptions.

Alternate flowers

Opposite flowers

Metatopy

When a single flower or a cluster is located at the axil of a bract (which can be reduced), the location of the bract in relation to the stem holding the flower(s) is indicated by the use of different terms and may be a useful diagnostic indicator.

• In concaulescence, the pedicel or peduncle is adnate or attached to the main stem and the bract is free from the attachment point.
• In recaulescence, the pedicel or peduncle is adnate or attached to the petiole of the bract.

Flower and subtending bract	Lilium martagon (flower and subtending bract)	Concaulescence	Solanum lycopersicum (concaulescence)
Recaulescence	Tilia cordata (recaulescence)

Organisation

It should be noted that there is no general consensus in defining the different inflorescences. The following is based on Focko Weberling's Morphologie der Blüten und der Blütenstände (Stuttgart, 1981).

The main groups of inflorescences are distinguished by branching. Within these groups, the most important characteristics are the intersection of the axes and different variations of the model.

Inflorescences can be simple or compound.

Simple inflorescences

Indeterminate simple inflorescences are generally called racemose. The main kind of racemose inflorescence is the raceme, the other kind of racemose inflorescences can all be derived from this one by dilation, compression, swelling or reduction of the different axes. Some passage forms between the obvious ones are commonly admitted.

• A raceme is an unbranched, indeterminate inflorescence with pedicellate (having short floral stalks) flowers along the axis.
• A spike is a type of raceme with flowers that do not have pedicel.
• A racemose corymb is a unbranched, indeterminate inflorescence that is flat-topped or convex because the outer pedicels are progressively longer than the inner ones.
• An umbel is a type of raceme with a short axis and multiple floral pedicels of equal length that appear to arise from a common point.
• A cone is an enlarged spike with simplified flowers protected by scaled. It is characteristic of the Pinaceae.^{[citation needed]} Pinaceae is a family of conifers, and not a flowering plant.
• A spadix is a spike of flowers densely arranged around it, enclosed or accompanied by a highly specialised bract called spathe. It is characteristic of the Araceae.
• A flower head or capitulum is a very contracted raceme in which the single sessile flowers share are borne on an enlarged stem. It is characteristic of the Dipsacaceae.
• A catkin or ament is a scaly, generally drooping spike or raceme. Cymose or other complex inflorescences that are superficially similar are also generally called thus.

Raceme	Epilobium angustifolium	Spike	Plantago media (spike)
Racemose corymb	Iberis umbellata (racemose corymb)	Umbel	Astrantia minor (umbel)
Cone	Larix decidua (cone)	Spadix	Arum maculatum (spadix)
Head (round)	Dipsacus fullonum (head)	Catkin (racemose or spicate)	Alnus incana (ament)

Determinate simple inflorescences are generally called cymose. The main kind of cymose inflorescence is the cyme. Cymes are further divided according to this scheme

• Only one secondary axis: monochasium
  • Secondary buds always develop on the same side of the stem: scorpioid cyme
    • The successive pedicels describe a spiral around the axis of the inflorescence: bostryx
    • The successive pedicels are aligned on the same plane: drepanium
  • Secondary buds develop alternately on the stem : helicoid cyme
    • The successive pedicels are arranged to describe a sort of spiral: cincinnus (characteristic of the Boraginaceae)
    • The successive pedicels follow a zigzag path on the same plane: rhipidium (many Iridaceae)
• Two secondary axes: dichasial cyme
  • Secondary axis still dichasial: dichasium (characteristic of the Caryophyllaceae)
  • Secondary axis monochasia: double scorpioid cyme or double helicoid cyme
• More than two secondary axes: pleiochasium

It should be noted that, although it is commonly said to be a scorpioid cyme, the most typical inflorescence of the family Boraginaceae is in fact the cincinnus, that is a helicoid cyme.

Dichasium	Dichasium, top view	Silene dioica (dichasium)	Double cyme
Double cyme	Cincinnus (lateral and top view)	Symphytum officinale (cincinnus)	Bostryx (lateral and top view)
Hypericum perforatum (bostryx)	Rhipidium (lateral and top view)	Canna sp. (rhipidium)	Drepanium (lateral and top view)
Gladiolus imbricatus (drepanium)

A cyme can also be so compressed that it looks like an umbel. Strictly speaking this kind of inflorescence could be called umbelliform cyme, although it is normally called simply 'umbel'.

Another kind of definite simple inflorescence is the raceme-like cyme or botryoid, that is as a raceme with a terminal flower and is usually improperly called 'raceme'.

Umbelliform cyme

Pelargonium zonale (umbelliform cyme)

Botryoid

Berberis vernae (botryoid)

A reduced raceme or cyme that grows in the axil of a leaf (actually bract) is called a fascicle. A verticillaster is a fascicle with the structure of a dichasium; it is common among the Lamiaceae. Many verticillasters with reduced bracts can form a spicate (spike-like) inflorescence that is commonly simply called spike.

Gentiana lutea (fascicles)

Lamium orvala (verticillaster)

Mentha longifolia ('spike')

Compound inflorescences

Simple inflorescences are the basis for compound inflorescences or synflorescences. The single flowers are there replaced by a simple inflorescence, which can be both a racemose or a cymose one. Compound inflorescences are composed of branched stems and can involve complicated arrangements that are difficult to trace back to the main branch.

A kind of compound inflorescence is the double inflorescence, in which the basic structure is repeated in the place of single florets. For example a double raceme is a raceme in which the single flowers are replaced by other simple racemes. Of course the same structure can be repeated to form also triple inflorescences or even more complex structures.

Compound racemes inflorescens can either end with a final raceme (homoeothetic) or not (heterothetic). A compound raceme is often called a panicle. Note that this definition is very different from that given by Weberling.

Compound umbles are umbels in which the single flowers are replaced by many smaller umbles called umbellets. The stem attaching the side umbellets to the main stem is called a ray.

Homeothetic compound raceme	Melilotus officinalis (homoeothetic compound raceme)	Heterothetic compound raceme	Hebe albicans (heterothetic compound raceme)
Compound spike	Lolium temulentum (compound spike)	Compound capitulum	Echinops ritro (compound capitulum)
Compound (double) umbel	Laserpicium latifolium (double umbel)	Compound (triple) umbel

The most common kind of definite compound inflorescence is the panicle (of Webeling, or 'panicle-like cyme'). A panicle is a definite inflorescence that is increasingly more strongly and irregularly branched from the top to the bottom and where each braching has a terminal flower.

The so called cymose corymb is similar to a racemose corymb but has a panicle-like structure. Another type of panicle is the anthela. An anthela is a cymose corymb with the lateral flowers higher than the central ones.

Panicle	Vitis vinifera (panicle)	Cymose corymb	Sambucus nigra (cymose corymb)
Anthela	Juncus inflexus (anthela)

A raceme in which the single flowers are replaced by cymes is called a (indefinite) thyrse. The secondary cymes can of course be of any of the differnt types of dichasia and monochasia. A botryoid in which the single flowers are replaced by cymes is a definite thyrse or thyrsoid. Also thyrses are often confusingly called panicles.

Thyrse

Aesculus hippocastanum

Thyrsoid

Syringa vulgaris

Other combinations are, of course, possible. For example, heads or umbels may be arranged in a corymb or a panicle.

Achillea sp. (heads in a corymb)

Hedera helix (umbels in a panicle)

Other

The family Asteraceae is characterised by a highly specialised head technically called calathid (but usually referred to as 'capitulum' or 'head'). The family Poaceae has a peculiar inflorescence of small spikes (spikelets) organised in panicles or spikes that are usually simply and improperly referred to as spike and panicle. The genus Ficus (Moraceae) has an inflorescence called syconium and the genus Euphorbia has cyathia (sing. cyathium), usually organised in umbels. For detailed descriptions, see the respective articles.

Matricaria chamomilla (calathid)	Triticum aestivum (compound spikes, "spikes")	Oryza sativa (spikes in a panicle, "panicle")	Ficus carica (syconium)
Euphorbia tridentata (cyathium)	Euphorbia cyparissias (cyathia in an umbel)

References and external links

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Inflorescence

This article is based on a translation of an article from the German Wikipedia.

• Focko Weberling: Morphologie der Blüten und der Blütenstände; Zweiter Teil. Verlag Eugen Ulmer, Stuttgart 1981
• Wilhelm Troll: Die Infloreszenzen; Erster Band. Gustav Fischer Verlag, Stuttgart 1964
• Wilhelm Troll: Die Infloreszenzen; Zweiter Band, Erster Teil. Gustav Fischer Verlag, Stuttgart 1969
• Wilhelm Troll: Praktische Einführung in die Pflanzenmorphologie. Gustav Fischer Verlag, Jena 1957
• Bernhard Kausmann: Pflanzenanatomie. Gustav Fischer Verlag, Jena 1963
• Walter S. Judd, Christopher S. Campbell, Elizabeth A. Kellogg, Peter F. Stevens, Michael J. Donoghue: Plant Systematics: A Phylogenetic Approach, Sinauer Associates Inc. 2007
• Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 7, May 2006 [and more or less continuously updated since]. [1]
• Strasburger, Noll, Schenck, Schimper: Lehrbuch der Botanik für Hochschulen. 4. Auflage, Gustav Fischer, Jena 1900, p. 459

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