The Inflorescence

The particular way in which flowers are arranged on the floral axis (also called the peduncle) is called the inflorescence. This arrangement is not incidental -- it reflects a fundamental difference in how the floral axis itself grows, and this difference divides inflorescences into two broad types: racemose and cymose.

Racemose inflorescence. In a racemose inflorescence, the main floral axis is indeterminate: its growing tip continues to elongate and go on producing new flowers laterally for as long as growing conditions permit, rather than being used up in forming a flower itself. As a direct consequence, the flowers along the axis are of different ages depending on their position -- the oldest, most fully developed flowers are found nearest the base of the axis (having been produced earliest), while the youngest flower buds are found nearest the still-actively-growing tip. This sequence, in which flowers mature progressively from the base of the axis towards its tip, is called acropetal succession, and it is the single most reliable test for recognising a racemose inflorescence. The mustard plant's flowering axis is a standard example: fully opened yellow flowers sit low on the axis, with progressively smaller, less developed buds visible higher up, right up to the still-elongating growing tip.

Cymose inflorescence. In a cymose inflorescence, by contrast, the main floral axis is determinate: its growing point is entirely used up in forming a single terminal flower, so the main axis itself stops growing further once that flower has formed. Any additional flowers the plant produces must then arise from lateral branches that develop below this terminal flower, and those lateral branches may themselves end in their own terminal flowers, with further branching continuing below them in turn. Because the very first flower to form is the terminal one (at or near the tip of the original axis), and subsequently produced flowers appear on the branches below it, the general sequence of flower maturity runs in the opposite direction to a raceme -- youngest near the base, on the newest branches, and oldest at or near the original tip. Jasmine is a standard textbook example of a cymose inflorescence. …