Botany Five - Inflorescences

A Beginner’s Guide to Botanical Terminology (Pt 5).

Botany and biological description are a bit more exacting than Latin nomenclature. Often there is only the correct, Latin-based name for a particular part of a plant, or animal. As a gardener you may not need to trouble with the finer details but if you get interested in how & why plants do what they do, or citizen science and recording the plants around you, you will need to know some of this to work with identification keys. A passing familiarity with Botanical Latin can also be advantageous, try our Botanical Latin Lexicon.

This page is only a starting point in to how and why plants do what they do. The framework for Botanical Descriptions has been used as starting point for items included and I hope that relating observable plant features and processes to the correct Biological terms makes this fascinating subject a little more accessible.

Inflorescences

→ Anatomy

→ Phyllotaxy & Determinance

→ Forms

After a juvenile period, flowering plants, (Angiosperms) bear flowers. Flowers are borne singly, termed Solitary, or in a cluster. Flowers borne in clusters, together with the associated stems and bracts are known as Inflorescences. (In plants with no or greatly reduced stems, usually annuals, e. g. Radishes, the flowering stem is termed a Scape).

Inflorescences have branching patterns particular to each species but have forms common across species. They are a way for a plant to maximise its reproductive success. Flowers collected into showy structures better attract pollinators, to increase seed production, and other animals to aid in seed dispersal.

Inflorescence Anatomy

Inflorescences are modified stems. When plants move from vegetative to reproductive growth the Axillary and / or Apical meristems transform into Inflorescence Meristems that in turn produce Floral Meristems. The exact structure of an Inflorescence depends on the relative dominance of these meristems and is influenced by the genetics of the species.

The Stem that holds the whole Inflorescence is termed the Peduncle, the major axis above the Peduncle, which bears the Flowers or Secondary branches is the Rachis, and the stalks of individual Flowers are Pedicels. Flowers with Pedicels are termed Pedicellate, those without are termed Sessile.

Bracts

Inflorescences often contain modified foliage known as Bracts. Bracts usually occur at Nodes, where the Peduncle meets the Rachis and where the Rachis meets the Pedicels. They can protect flower buds and help attract pollinators. The presence and style of the Bracts give rise to four types of Inflorescence.

- - - Ebracteate inflorescences – Have no bracts.
    - Bracteate inflorescences – Bracts in the inflorescence are specialised, sometimes reduced to small scales, divided, or dissected.
    - Leafy inflorescences – Bracts are unspecialised, often look like the typical leaves of the plant though reduced in size. Despite their ‘normal’ appearance, these “leaves” are considered Bracts.
    - Leafy-bracted inflorescences – Intermediate between bracteate and leafy inflorescence.

When many bracts are present and strictly connected to the stem, 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. Flowers with Bracts are called Bracteate and without a Bract are called Ebracteate.The exact placement and attachment style of Bracts can be an important diagnostic character between plant species. There are three attachment styles.

- - - Subtending Bract – Where Flowers arise on Pedicals the Bract is not attaced to the Pedicel but it Adnate (fused to) the Rachis / Peduncle below the Pedicel.
    - Concaulescent Bract – Where the bud formation is distinctly above the subtending Bract.
    - Recaulescent Bract – Where the Bract is attached to the Pedicel / Peduncle bearing the Flower

Phyllotaxy & Determinacies

Phyllotaxy in Inflorescences is similar to that in leaves, there are two main forms – Alternate and Opposite. For more on Pyhllotaxy in general see Here, and as Inflorecences are modified Stems see Here for Stem Branching.

Determinance – Plant organs grow in two different schemes, with Inflorescences –

Indeterminate Inflorescences – exhibit Monopodial or Racemose growth. The terminal bud keeps growing and forms lateral Flowers which mature from the oldest first. No Terminal Flower is produced.
Determinate Inflorescences – exhibit Symopodial or Cymose growth. The terminal bud forms a Terminal Flower then dies out. Lateral buds then develop into more Flowers which mature from the oldest first.

Inflorescence Forms

Inflorescences may contain many flowers (Pluriflor), or a few (Pauciflor). Forms are usually grouped together by virtue of their growth styles and whether they are Simple or Compound.

Indeterminate Simple Inflorescences – are generally referred to as Racemose. The base form is the Raceme with variations being formed by swelling or reducing the various axes.

• Raceme – an unbranched, indeterminate inflorescence with Pedicellate Flowers (on short stalks) along its axis.
• Spike – a Raceme with Flowers without Pedicels.
• Corymb – (Racemose Corymb) Convex or flat-topped Raceme with outer pedicels progressively longer than the center.
• Umbel – a Raceme with a very short axis and all Pedicels the same length. Characteristic of the Umbelliferae family.
• Spadix – a dense Spike of flowers accompanied by a larg Bract termed a Spathe. Characteristic of the family Aracea.
• Head – (Flower Head / Capitulum) A contracted Raceme with Sessile Flowers on an enlarged stem. Characteristic of Dipsacacea family e. g. Teasel.
• Catkin – (Amet) a scaly and generally drooping Raceme of Spike.

Determinate Simple Inflorescences – are generally referred to as Cymose. The base form is the Cyme, further forms as divided as follows –

Monochasium – only one secondary axis.
- Bostryx- (Helicoid Cyme), secondary buds develop on the same side of the Stem.
  - Drepanium – Successive Pedicels aligned in the same plane.
- Scorpioid Cyme – secondary buds develop alternately on the Stem.
  - Cincinnus – successive Pedicels arranged in a spiral. Characteristic of Boraginacea.
  - Rhipidium – successive Pedicels arranged on the same plane in a zig-zag path. Characteristic of Iridacea.
Dichasial Cyme – two secondary axes.
- Dichasium – secondary axis still dichasial. Characteristic of Careophllacea.
- Double Heliciod Cyme – secondary axis monochasial, or
- Double Scorpioid Cyme – secondary axis monochasial.
Pleiochasium – more than two secondary axes.

There are two other forms of Cyme –

Umbelliform Cyme – an arrangement of Cymes so compressed it appears as if am Umbel.
Botryoid – An arrangement of stacked, stretched Cymes, with a terminal flower.

Compound Inflorescences – (Synflorescences) All of the simple inflorescences can be further modified by replacing single flowers of the simple types with further simple inflorescences. One iteration of this gives a Double Inflorescence, two would give a Triple Inflorescence, etc. Compound raceme (indeterminate) inflorescences can either end with a final raceme (Homoeothetic), or not (Heterothetic). The resulting complex inflorescences can be very difficult to identify.

← Botany Four – Phyllotaxy & Leaves

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Botany Six – Flowers→

Further Resources

Most of the main search engines will give you a result if you are looking for the meaning of a particular botanical word. If you wish to explore the subject in more detail here are a few website links to give you a starting point.