Inflorescence

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Aloe hereroensis, showing inflorescence with branched peduncle Aloe hereroensis Auob C15.JPG
Aloe hereroensis , showing inflorescence with branched peduncle

An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. [1] Morphologically, it is the modified part of the shoot of seed plants where flowers are formed. 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. One can also define an inflorescence as the reproductive portion of a plant that bears a cluster of flowers in a specific pattern.

Contents

The stem holding the whole inflorescence is called a peduncle and the major axis (incorrectly referred to as 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 also referred to as a peduncle. Any flower in an inflorescence may be referred to as a floret, especially when the individual flowers are particularly small and borne in a tight cluster, such as in a pseudanthium. The fruiting stage of an inflorescence is known as an infructescence. Inflorescences may be simple (single) or complex (panicle). The rachis may be one of several types, including single, composite, umbel, spike or raceme.

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 it. These terms are general representations as plants in nature can have a combination of types. These structural types are largely based on natural selection. [2]

Bracts

Inflorescences usually have modified shoots 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 where the main stem of the inflorescence forms, joined to the main stem of the plant, but other bracts can exist within the inflorescence itself. They serve a variety of functions which include attracting pollinators and protecting young flowers. According to the presence or absence of bracts and their characteristics we can distinguish:

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.

Terminal flower

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

Indeterminate and determinate inflorescences are sometimes referred to as open and closed 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. [3]

In an 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.

In determinate inflorescences the terminal flower is usually the first to mature (precursive 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.

Phyllotaxis

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

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:

Metatopic placement of bracts include:

Organization

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. They may contain many flowers (pluriflor) or a few (pauciflor). Inflorescences can be simple or compound.

Simple inflorescences

Inflorescence of sessile disc florets forming the capitulum Sunflower macro wide.jpg
Inflorescence of sessile disc florets forming the capitulum

Indeterminate or racemose

Indeterminate simple inflorescences are generally called racemose /ˈræsɪms/ . The main kind of racemose inflorescence is the raceme ( /ˈræsm/ , from classical Latin racemus, cluster of grapes). [5] 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 a pedicel.
  • A racemose corymb is an unbranched, indeterminate inflorescence that is flat-topped or convex due to their outer pedicels which are progressively longer than 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. It is characteristic of Umbelliferae.
  • A spadix is a spike of flowers densely arranged around it, enclosed or accompanied by a highly specialised bract called a spathe . It is characteristic of the family 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 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.

Determinate or cymose

Determinate simple inflorescences are generally called cymose. The main kind of cymose inflorescence is the cyme (pronounced 'saim', from the Latin cyma in the sense ‘cabbage sprout’, from Greek kuma ‘anything swollen’ [6] ). [7] 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 or bostryx
      • The successive pedicels are aligned on the same plane: drepanium
    • Secondary buds develop alternately on the stem : scorpoid cyme
      • The successive pedicels are arranged in a sort of spiral: cincinnus (characteristic of the Boraginaceae and Commelinaceae)
      • The successive pedicels follow a zig-zag path on the same plane: rhipidium (many Iridaceae)
  • Two secondary axes: dichasial cyme
    • Secondary axis still dichasial: dichasium (characteristic of Caryophyllaceae)
    • Secondary axis monochasia: double scorpioid cyme or double helicoid cyme
  • More than two secondary axes: pleiochasium

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'.

A reduced raceme or cyme that grows in the axil of a 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 called a 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; 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 a panicle. Note that this definition is very different from that given by Weberling.

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

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 branching 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.

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 a definite thyrse or thyrsoid. Thyrses are often confusingly called panicles.

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

Other

The family Asteraceae is characterised by a highly specialised head technically called a 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.

Development and patterning

Development

Genetic basis

Genes that shape inflorescence development have been studied at great length in Arabidopsis . LEAFY (LFY) is a gene that promotes floral meristem identity, regulating inflorescence development in Arabidopsis. [8] Any alterations in timing of LFY expression can cause formation of different inflorescences in the plant. [9] Genes similar in function to LFY include APETALA1 (AP1). Mutations in LFY, AP1, and similar promoting genes can cause conversion of flowers into shoots. [8] In contrast to LEAFY, genes like terminal flower (TFL) support the activity of an inhibitor that prevents flowers from growing on the inflorescence apex (flower primordium initiation), maintaining inflorescence meristem identity. [10] Both types of genes help shape flower development in accordance with the ABC model of flower development. Studies have been recently conducted or are ongoing for homologs of these genes in other flower species.

Environmental influences

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. [11] In the absence of this herbivory, inflorescences usually produce more flower heads and seeds. [11] 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. [12] [13] [14]

Meristems and inflorescence architecture

The shift from the vegetative to reproductive phase of a flower involves the development of an inflorescence meristem that generates floral meristems. [15] Plant inflorescence architecture depends on which meristems becomes flowers and which become shoots. [16] 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. [15]

On a larger scale, inflorescence architecture affects the 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. [17] In Aesculus sylvatica , it has been shown that the most common inflorescence sizes are correlated with the highest fruit production as well. [18]

Issues with inflorescence identification

Some species have flower and inflorescence intermediates. In these cases, some reproductive structures of certain flowers appear as transitional between inflorescences and flowers, making it difficult to accurately categorize and identify the structure as one or the other. For example, plants of the genus Potamogeton have inflorescences that appear to be single flowers. [19]

Related Research Articles

Panicle a compound raceme, occurring esp in grasses

A panicle is a much-branched inflorescence. Some authors distinguish it from a compound spike inflorescence, by requiring that the flowers be pedicellate. The branches of a panicle are often racemes. A panicle may have determinate or indeterminate growth.

Meristem Type of plant tissue involved in cell proliferation

The meristem is a type of tissue found in plants. It consists of undifferentiated cells capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants.

Malvaceae Family of flowering plants

Malvaceae, or the mallows, is a family of flowering plants estimated to contain 244 genera with 4225 known species. Well-known members of economic importance include okra, cotton, cacao and durian. There are also some genera containing familiar ornamentals, such as Alcea (hollyhock), Malva (mallow) and Lavatera, as well as Tilia. The largest genera in terms of number of species include Hibiscus, Sterculia, Dombeya, Pavonia and Sida.

Raceme Unbranched, indeterminate type of inflorescence bearing pedicellate flowers along its axis

A raceme or racemoid is an unbranched, indeterminate type of inflorescence bearing pedicellate flowers along its axis. In botany, an axis means a shoot, in this case one bearing the flowers. In indeterminate inflorescence-like racemes, the oldest flowers are borne towards the base and new flowers are produced as the shoot grows, with no predetermined growth limit. A plant that flowers on a showy raceme may have this reflected in its scientific name, e.g. Cimicifuga racemosa. A compound raceme, also called a panicle, has a branching main axis. Examples of racemes occur on mustard and radish plants.

Bract modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale

In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis or cone scale. Bracts are often different from foliage leaves. They may be smaller, larger, or of a different color, shape, or texture. Typically, they also look different from the parts of the flower, such as the petals or sepals. The state of having bracts is referred to as bracteate or bracteolate, and conversely the state of lacking them is referred to as ebracteate and ebracteolate, without bracts.

<i>Pulmonaria</i> Genus of flowering plants in the borage family Boraginaceae

Pulmonaria (lungwort) is a genus of flowering plants in the family Boraginaceae, native to Europe and western Asia, with one species east to central Asia. According to various estimates there may be between 10 and 18 species found in the wild.

<i>Commelina</i> Genus of flowering plants

Commelina is a genus of approximately 170 species commonly called dayflowers due to the short lives of their flowers. They are less often known as widow's tears. It is by far the largest genus of its family, Commelinaceae. The Swedish taxonomist Carl Linnaeus of the 18th century named the genus after the two Dutch botanists Jan Commelijn and his nephew Caspar, each representing one of the showy petals of Commelina communis.

Sympodial branching

Sympodial growth is type of bifurcating branching pattern where one branch develops more strongly than the other, resulting in the stronger branches forming the primary shoot and the weaker branches appearing laterally. A sympodium, also referred to as a sympode or pseudaxis, is the primary shoot, comprising the stronger branches, formed during sympodial growth. The pattern is similar to dichotomous branching; it is characterized by branching along a stem or hyphae.

<i>Aralia spinosa</i> species of plant

Aralia spinosa, commonly known as devil's walking stick, is a woody species of plant in the genus Aralia, family Araliaceae, native to eastern North America. The various names refer to the viciously sharp, spiny stems, petioles, and even leaf midribs. It has also been known as Angelica-tree.

Corymb an inflorescence in the form of a flat-topped flower cluster with the oldest flowers at the periphery. This type of raceme occurs in the candytuft

Corymb is a botanical term for an inflorescence with the flowers growing in such a fashion that the outermost are borne on longer pedicels than the inner, bringing all flowers up to a common level. A corymb has a flattish top with a superficial resemblance towards an umbel, and may have a branching structure similar to a panicle. Flowers in a corymb structure can either be parallel, or alternate, and form in either a convex, or flat form.

In botany, an umbel is an inflorescence that consists of a number of short flower stalks which spread from a common point, somewhat like umbrella ribs. The word was coined in botanical usage in the 1590s, from Latin umbella "parasol, sunshade". The arrangement can vary from being flat-topped to almost spherical. Umbels can be simple or compound. The secondary umbels of compound umbels are known as umbellules or umbellets. A small umbel is called an umbellule. The arrangement of the inflorescence in umbels is referred to as umbellate, or occasionally subumbellate.

Indeterminate growth

In biology and botany, indeterminate growth is growth that is not terminated in contrast to determinate growth that stops once a genetically pre-determined structure has completely formed. Thus, a plant that grows and produces flowers and fruit until killed by frost or some other external factor is called indeterminate. For example, the term is applied to tomato varieties that grow in a rather gangly fashion, producing fruit throughout the growing season, and in contrast to a determinate tomato plant, which grows in a more bushy shape and is most productive for a single, larger harvest, then either tapers off with minimal new growth or fruit, or dies.

This page provides a glossary of plant morphology. Botanists and other biologists who study plant morphology use a number of different terms to classify and identify plant organs and parts that can be observed using no more than a handheld magnifying lens. This page provides help in understanding the numerous other pages describing plants by their various taxa. The accompanying page—Plant morphology—provides an overview of the science of the external form of plants. There is also an alphabetical list: Glossary of botanical terms. In contrast, this page deals with botanical terms in a systematic manner, with some illustrations, and organized by plant anatomy and function in plant physiology.

<i>Polyscias aemiliguineae</i> species of plant

Polyscias aemiliguineae is a species of plant in the family Araliaceae. It is endemic to Réunion.

This glossary of botanical terms is a list of definitions of terms and concepts relevant to botany and plants in general. Terms of plant morphology are included here as well as at the more specific Glossary of plant morphology and Glossary of leaf morphology. For other related terms, see Glossary of phytopathology and List of Latin and Greek words commonly used in systematic names.

<i>Corymbium</i> Genus of perennial plants in the family Asteraceae from South Africa

Corymbium is a genus of flowering plants in the daisy family comprising nine species. It is the only genus in the subfamily Corymbioideae and the tribe Corymbieae. The species have leaves with parallel veins, strongly reminiscent of monocots, in a rosette and compounded inflorescences may be compact or loosely composed racemes, panicles or corymbs. Remarkable for species in the daisy family, each flower head contains just one, bisexual, mauve, pink or white disc floret within a sheath consisting of just two large involucral bracts. The species are all endemic to the Cape Floristic Region of South Africa, where they are known as plampers.

Adenodaphne is a genus of shrubs and small trees endemic to New Caledonia belonging to the family Lauraceae. The genus is related to Litsea. They have 12 chromosomes.

<i>Eucomis bicolor</i> species of plant

Eucomis bicolor, the variegated pineapple lily or just pineapple lily, is a bulbous species of flowering plant in the family Asparagaceae, subfamily Scilloideae, native to Southern Africa. The pale green, purple-margined flowers are arranged in a spike (raceme), topped by a "head" of green leaflike bracts. It is cultivated as an ornamental bulbous plant, although its flowers have an unpleasant smell, attractive to the main pollinators, flies.

<i>Hypericum majus</i> species of plant

Hypericum majus, the greater Canadian St. John's wort, is a perennial herb native to North America. The specific epithet majus means "larger". The plant has a diploid number of 16.

<i>Mimetes palustris</i> A shrub in the family Proteaceae endemic to South Africa

Mimetes palustris or cryptic pagoda is an evergreen shrub, assigned to the family Proteaceae. It has horizontal sprawling shoots as well as upright, unbranched shoots usually about ½ m high. The leaves are entire and stand out on the lower parts of the shoots, but are overlapping and pressed tidly against each other near the inflorescence, almost like a snakeskin. The inflorescence consists of several flowerheads, each containing three clear yellow flowers that are longer than the subtending leaves. It is the smallest species of Mimetes and is an endemic species that grows on well-drained, but permanently moist sandy and peaty slopes in the mountains near Hermanus, Western Cape province of South Africa. It is considered critically endangered. Flowering occurs all year round, but peaks in August and September.

References

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  6. Collins English Dictionary. 8th Edition first published in 2006
  7. 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 definite inflorescence: opposed to Raceme. Applied esp. to compound inflorescences of this type forming a more or less flat head.
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