Types of Inflorescences

Biol 324 Introduction To Seed Plant Taxonomy
Biol 324 Course topics


Inflorescences are aggregations of flowers. Plants have different "inflorescence architectures", i.e. different ways of grouping of flowers on the plant.

Two Fundamentally Different Types:
- Indeterminate, with an axis that continues growing (racemose inflorescences, racemes)
- Determinate, with a flower terminating the axis (cymose inflorescences, cymes)

Another Axis of Variation - Branchiness:
- unbranched, i.e. simple inflorescences
- branched, i.e. compound inflorescences, branching

Four main categories of inflorescence in flowering plants:
1. Single terminal flower (one extreme)
- no branching, complete determinacy
Example: Magnolia.

2. Raceme: indeterminate, unbranched
- new flowers are generated at the tip of the inflorescence, no definite determination, never terminate in a flower
Example: Antirrhinum majus (snapdragon)

3. Compound cyme: determinate, branched
- axis terminates in a flower, lateral floral branches develop below terminal flower, each branch ends in a flower, but produce lateral branches too. Every axis terminated in a flower
Example: Silene dioica (red campion)

4. Panicle (compound raceme): branched, indeterminate
Example: Vitis (grape)

There is some variation on these patterns. Below is a conspectus of the terms used to describe inflorescences.

A. Unbranched racemose inflorescences with pedicellate flowers
- Raceme - elongate, unbranched, indeterminate, e.g. the inflorescence of snapdragon (Antirrhinum)
- Corymb - a flat topped or convex indeterminate unbranched cluster, i.e., a flat topped raceme.
- Umbel - flat topped or convex inflorescence with all pedicels arising from a common point. It looks like the spokes of an umbrella.

B. Unbranched racemose inflorescences with sessile flowers
- Spike - elongate, unbranched, indeterminate
- Ament or catkin - a spike of unisexual, apetalous flowers shed as a unit- wind pollination
- Capitulum or head - A crowded group of sessile flowers on an enlarged compound receptacle. The whole inflorescence functions as a single flower for pollinator attraction - e.g. Asteraceae.
- Spadix - unbranched, indeterminate inflorescence with flowers embedded in the fleshy stem (rachis), usually surrounded or subtended by a spathe, a bract – e.g. Araceae

C. Branched racemose inflorescences
- Compound umbel - a branched umbel, composed of several umbels united into a bigger umbel. Example: Sambucus nigra - elderberry
- Panicle - a branched, indeterminate inflorescence with pedicellate flowers.

D. Mixed inflorescences
- Umbellate Cyme – central flower subtended by umbellate branching, initial dichasium succeeded by monochasia
- Determinate Raceme – raceme, with terminal flower. Example: blackberry (Rubus)

E. Cymes: main axis immediately terminated by flower, but branching
- Dichasium (or simple cyme) - a determinate inflorescence with a terminal flower that opens first, and two opposite flowers below it (three flowers in all).
- Compound dichasium or compound cyme (sometimes just called a cyme) - a branched cyme, determinate with each ultimate unit three flowered (more than three flowers).
- Monochasial cyme (terminal flower with one flower below). This may repeat (branch) many times to give a long coiled inflorescence (helicoid cyme). The one branch can go in different directions (see diagram below)

Types of monochasial cymes:
Boraginaceae, two rows of flowers – commonly known monochasium, cincinnus in Boraginaceae (forget-me-not)
Note: In lab you will come across terms which apply to the general form of the cincinnus: helicoid cymes/scorpioid cymes refer to how the inflorescence coils

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