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
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