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. 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.
1 General characteristics
1.1 Bracts 1.2 Terminal flower 1.3 Phyllotaxis 1.4 Metatopy
2.1 Simple inflorescences
2.1.1 Indeterminate or racemose 2.1.2 Determinate or cymose
2.2 Compound inflorescences 2.3 Other
3 References 4 Bibliography 5 External links
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. 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:
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. 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 of Wisteria sinensis
Bracteate inflorescence of Pedicularis verticillata.
Leafy-bracted inflorescence of Rhinanthus angustifolius.
Leafy inflorescence of Aristolochia clematitis.
Indeterminate and determinate inflorescences are sometimes referred to as open and closed inflorescences respectively. 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.
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)
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.
Determinate inflorescence with acropetal maturation
Determinate inflorescence with basipetal maturation
Determinate inflorescence with divergent maturation
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. Metatopy Metatopy is the placement of organs out of their normally expected position: typically metatopy occurs in inflorescences when unequal growth rates alter different areas of the axis and the organs attached to the axis. 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 are adnate 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 the pedicel 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 be recaulescent; 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, 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 as concaulescent.
Solanum lycopersicum (concaulescence)
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
Indeterminate or racemose Indeterminate simple inflorescences are generally called racemose /ˈræsɪmoʊs/. The main kind of racemose inflorescence is the raceme (/ˈræsiːm/, from classical Latin racemus, cluster of grapes). 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.
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
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’). 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 : scorpioid cyme
The successive pedicels are arranged in a sort of spiral: cincinnus
(characteristic of the
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
Bostryx (lateral and top view)
Drepanium (lateral and top view)
Gladiolus imbricatus (drepanium)
Cincinnus (lateral and top view)
Rhipidium (lateral and top view)
Canna sp. (rhipidium)
Dichasium, top view
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'.
Pelargonium zonale (umbelliform cyme)
Berberis vernae (botryoid)
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.
Homeothetic compound raceme
Heterothetic compound raceme
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 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.
Juncus inflexus (anthela)
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.
^ 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.
Focko Weberling: Morphologie der Blüten und der Blütenstände;
Zweiter Teil. Verlag Eugen Ulmer, Stuttgart 1981
Wilhelm Troll: Die Infloreszenzen; Erster Band. Gustav Fischer Verlag,
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:
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