Inbotany, aninflorescence is a group or cluster offlowers arranged on aplant'sstem that is composed of a mainbranch or a system of branches.[1] An inflorescence is categorized on the basis of the arrangement of flowers on a main axis (peduncle) and by the timing of its flowering (determinate and indeterminate).[2]
Morphologically, an inflorescence is the modified part of theshoot ofseed plants where flowers are formed on the axis of a plant. The modifications can involve the length and the nature of theinternodes and thephyllotaxis, as well as variations in the proportions, compressions, swellings,adnations,connations and reduction of main and secondary axes.[citation needed]
One can also define an inflorescence as the reproductive portion of a plant that bears a cluster of flowers in a specific pattern.[citation needed]
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 it. These terms are general representations as plants in nature can have a combination of types. Because flowers facilitateplant reproduction, inflorescence characteristics are largely a result ofnatural selection.[3]
The stem holding the whole inflorescence is called apeduncle. The main axis (also referred to as major stem) above the peduncle bearing the flowers or secondary branches is called therachis. The stalk of each flower in the inflorescence is called apedicel. A flower that is not part of an inflorescence is called a solitary flower and its stalk is also referred to as a peduncle. Any flower in an inflorescence may be referred to as afloret, especially when the individual flowers are particularly small and borne in a tight cluster, such as in apseudanthium. Thefruiting stage of an inflorescence is known as aninfructescence. Inflorescences may be simple (single) or complex (panicle). The rachis may be one of several types, including single, composite, umbel, spike orraceme.[citation needed]
In some species the flowers develop directly from the main stem or woody trunk, rather than from the plant's main shoot. This is calledcauliflory and is found across a number of plant families.[4] An extreme version of this isflagelliflory where long, whip-like branches grow from the main trunk to the ground and even below it. Inflorescences form directly on these branches.[5]
Plant organs can grow according to two different schemes, namelymonopodial orracemose andsympodial orcymose. In inflorescences these two different growth patterns are calledindeterminate and determinate respectively, and indicate whether a terminal flower is formed and where flowering starts within the inflorescence.
Indeterminate inflorescence:Monopodial (racemose) growth. The terminal bud keeps growing and forming lateral flowers. A terminal flower is never formed.
Determinate inflorescence:Sympodial (cymose) 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 asopen andclosed inflorescences respectively. The indeterminate patterning of flowers is derived from determinate flowers. It is suggested that indeterminate flowers have a common mechanism that prevents terminal flower growth. Based on phylogenetic analyses, this mechanism arose independently multiple times in different species.[6]
In anindeterminate 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 an acropetal maturation and lateral flower buds
Indeterminate inflorescence with the subterminal flower to simulate the terminal one (vestige present)
Indeterminate inflorescences the terminal flower is usually the first to mature (precursive development), while the others tend to mature starting from the base of the stem. This pattern is calledacropetal maturation. When flowers start to mature from the top of the stem, maturation isbasipetal, whereas when the central mature first, maturation isdivergent.
Determinate inflorescence with acropetal maturation
Determinate inflorescence with basipetal maturation
Determinate inflorescence with divergent maturation
As withleaves, flowers can be arranged on the stem according to many different patterns. See 'Phyllotaxis' for in-depth descriptions.
Alternate flowers
Opposite flowers
Similarly arrangement of leaf in bud is called Ptyxis.
When a single or a cluster of flower(s) is located at the axil of a bract, 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.
Typical placement of bracts include:
Some plants have bracts that subtend the inflorescence, where the flowers are on branched stalks; the bracts are not connected to the stalks holding the flowers, but areadnate or attached to the main stem (Adnate describes the fusing together of different unrelated parts. When the parts fused together are the same, they are connately joined.)
Other plants have the bracts subtend thepedicel or peduncle of single flowers.
Metatopic placement of bracts include:
When the bract is attached to the stem holding the flower (the pedicel or peduncle), it is said to berecaulescent; sometimes these bracts or bracteoles are highly modified and appear to be appendages of the flower calyx. Recaulescences is the fusion of the subtending leaf with the stem holding the bud or the bud itself,[7] thus the leaf or bract is adnate to the stem of flower.
When the formation of the bud is shifted up the stem distinctly above the subtending leaf, it is described asconcaulescent.
There is no general consensus in defining the different inflorescences. The following is based onFocko Weberling'sMorphologie 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. They may contain many flowers (pluriflor) or a few (pauciflor). Inflorescences can besimple orcompound.
Indeterminate simple inflorescences are generally calledracemose/ˈræsɪmoʊs/. The main kind of racemose inflorescence is theraceme (/ˈræsiːm/, from classical Latinracemus,cluster of grapes).[8] 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.
Araceme is an unbranched,indeterminate inflorescence with pedicellate (having short floral stalks) flowers along the axis.
Aspike is a type of raceme with flowers that do not have a pedicel.
A racemosecorymb is an unbranched, indeterminate inflorescence that is flat-topped or convex due to their outer pedicels which are progressively longer than inner ones.
Anumbel is a type of raceme with a short axis and multiple floral pedicels of equal length that appear to arise from a common point. It is characteristic ofUmbelliferae.
Aspadix is a spike of flowers densely arranged around it, enclosed or accompanied by a highly specialised bract called aspathe. It is characteristic of the familyAraceae.
Aflower head orcapitulum is a very contracted raceme in which the single sessile flowers share are borne on an enlarged stem. It is characteristic ofDipsacaceae.
Acatkin orament is a scaly, generally drooping spike or raceme. Cymose or other complex inflorescences that are superficially similar are also generally called thus.
Determinate simple inflorescences are generally calledcymose. The main kind of cymose inflorescence is thecyme (pronounced/saɪm/), from the Latincyma in the sense 'cabbage sprout', from Greekkuma 'anything swollen').[9][10] Cymes are further divided according to this scheme:
Only one secondary axis:monochasium
Secondary buds always develop on the same side of the stem:helicoid cyme orbostryx
The successive pedicels are aligned on the same plane:drepanium
Secondary buds develop alternately on the stem :scorpioid cyme
The successive pedicels are arranged in a sort of spiral:cincinnus (characteristic of theBoraginaceae andCommelinaceae)
The successive pedicels follow a zig-zag path on the same plane:rhipidium (manyIridaceae)
Two secondary axes:dichasial cyme
Secondary axis still dichasial:dichasium (characteristic ofCaryophyllaceae)
A cyme can also be so compressed that it looks like an umbel. Strictly speaking this kind of inflorescence could be calledumbelliform cyme, although it is normally called simply 'umbel'.
Another kind of definite simple inflorescence is the raceme-like cyme orbotryoid; that is as a raceme with a terminal flower and is usually improperly called 'raceme'.
A reduced raceme or cyme that grows in theaxil of a bract is called afascicle. Averticillaster is a fascicle with the structure of a dichasium; it is common among theLamiaceae. Many verticillasters with reduced bracts can form a spicate (spike-like) inflorescence that is commonly called aspike.
Simple inflorescences are the basis for compound inflorescences orsynflorescences. 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 thedouble 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; the same structure can be repeated to form triple or more complex structures.
Compound raceme inflorescences can either end with a final raceme (homoeothetic), or not (heterothetic). A compound raceme is often called apanicle. This definition is very different from that given byWeberling.
Compound umbels are umbels in which the single flowers are replaced by many smaller umbels calledumbellets. The stem attaching the side umbellets to the main stem is called aray.
The most common kind of definite compound inflorescence is thepanicle (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 branching has a terminal flower.
The so-called cymosecorymb is similar to a racemose corymb but has a panicle-like structure. Another type of panicle is theanthela. An anthela is a cymose corymb with the lateral flowers higher than the central ones.
A raceme in which the single flowers are replaced by cymes is called a (indefinite)thyrse. The secondary cymes can be of any of the different types of dichasia and monochasia. A botryoid in which the single flowers are replaced by cymes is adefinite thyrse orthyrsoid. Thyrses are often confusingly calledpanicles.
The familyAsteraceae is characterised by a highly specialised head technically called acalathid (but usually referred to as 'capitulum' or 'head'). The familyPoaceae has a peculiar inflorescence of small spikes (spikelets) organised in panicles or spikes that are usually simply and improperly referred to asspike andpanicle. The genusFicus (Moraceae) has an inflorescence called a hypanthodium, which bears numerous flowers on the inside of a convex or involuted compound receptacle.[11] The genusEuphorbia hascyathia (sing.cyathium), usually organised in umbels.
Some species have inflorescences reduced tocomposite flowers or pseudanthia, in which case it is difficult to differentiate between inflorescences and single flowers.[12]
Genes that shape inflorescence development have been studied at great length inArabidopsis.LEAFY (LFY) is a gene that promotesfloral meristem identity, regulating inflorescence development inArabidopsis.[13] Any alterations in timing of LFY expression can cause formation of different inflorescences in the plant.[14] Genes similar in function to LFY includeAPETALA1 (AP1). Mutations in LFY, AP1, and similar promoting genes can cause conversion of flowers into shoots.[13] In contrast to LEAFY, genes liketerminal flower (TFL) support the activity of an inhibitor that prevents flowers from growing on the inflorescence apex (flower primordium initiation), maintaining inflorescence meristem identity.[15] Both types of genes help shape flower development in accordance with theABC model of flower development. Studies have been recently conducted or are ongoing for homologs of these genes in other flower species.
Inflorescence-feeding insect herbivores shape inflorescences by reducing lifetime fitness (how much flowering occurs), seed production by the inflorescences, and plant density, among other traits.[16] In the absence of these herbivores, inflorescences usually produce more flower heads and seeds.[16] Temperature can also variably shape inflorescence development. High temperatures can impair the proper development of flower buds or delay bud development in certain species, while in others an increase in temperature can hasten inflorescence development.[17][18][19]
The shift from the vegetative to reproductive phase of a flower involves the development of an inflorescence meristem that generates floral meristems.[20] Plant inflorescence architecture depends on which meristems becomes flowers and which become shoots.[21] Consequently, genes that regulate floral meristem identity play major roles in determining inflorescence architecture because their expression domain will direct where the plant's flowers are formed.[20]
On a larger scale, inflorescence architecture affects quality and quantity of offspring from selfing and outcrossing, as the architecture can influence pollination success. For example,Asclepias inflorescences have been shown to have an upper size limit, shaped by self-pollination levels due to crosses between inflorescences on the same plant or between flowers on the same inflorescence.[22] InAesculus sylvatica, it has been shown that the most common inflorescence sizes are correlated with the highest fruit production as well.[23]
^Kubitzki, Klaus, and Clemens Bayer. 2002.Flowering plants, Dicotyledons: Malvales, Capparales, and non-betalain Caryophyllales. The Families and genera of vascular plants, 5. Berlin: Springer. p. 77
^Oxford English Dictionary. Raceme 2. Bot. A type of inflorescence in which the flowers are arranged on short, nearly equal, lateral pedicels, at equal distances along a single elongated axis
^Collins English Dictionary. 8th Edition first published in 2006
^Oxford English Dictionary. Cyme(1) Bot. A species of inflorescence wherein the primary axis bears a single terminal flower which develops first, the system being continued by axes of secondary and higher orders which develop successively in like manner; a centrifugal or definiteinflorescence: opposed toRaceme. Applied esp. to compound inflorescences of this type forming a more or less flat head.
^Simpson, M. G. 2010.Plant Systematics Burlington: Academic Press. p. 509
^Moss, G (27 November 2015). "Influence of Temperature and Photoperiod on Flower Induction and Inflorescence Development in Sweet Orange (Citrus Sinensis L. Osbeck)".Journal of Horticultural Sciences.44 (4):311–320.doi:10.1080/00221589.1969.11514314.
^BREWSTER, J. L. (1983-04-01). "Effects of Photoperiod, Nitrogen Nutrition and Temperature on Inflorescence Initiation and Development in Onion (Allium cepa L.)".Annals of Botany.51 (4):429–440.doi:10.1093/oxfordjournals.aob.a086487.ISSN0305-7364.
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
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