Why some flower spikes keep growing forever while others stop at one bloom
Look at the tip of a flower cluster and you can read its growth plan. Some stems never finish with a flower, their growing tip adding blooms along the sides indefinitely. Others cap themselves with a single flower first, then sprout the rest from lower buds. Botanists call these two strategies indeterminate and determinate.
An inflorescence is simply a flower cluster, the reproductive part of a shoot organised into a pattern. Its vocabulary sounds fussy but is logical. The stalk carrying the whole cluster is the peduncle, the axis running through the flowers is the rachis, and each flower's own little stalk is a pedicel. Once the flowers turn to fruit, the whole structure gets a new name, infructescence.
In an open, indeterminate cluster, the tip bud keeps growing and never forms a terminal flower, although the last flower often straightens up and pretends to be one. Flowers usually open from the bottom upward. The classic form is the raceme, named from the Latin for a bunch of grapes. Remove the flower stalks and you have a spike; shrink the axis until equal stalks radiate from one point and you get an umbel, typical of the carrot family. A spadix, the dense spike of arums, comes wrapped in a hood-like bract, and catkins are drooping, scaly spikes.
In a closed, determinate cluster, the tip makes a flower first and then stops, so the central bloom usually opens earliest. The basic form is the cyme, from a Latin word for a cabbage sprout. Cymes branch in varied ways, spiralling, zig-zagging or forking in pairs, which gives them names like scorpioid cyme. Evolutionary studies suggest the ability to stop a terminal flower forming arose independently several times.
Some plants break the usual layout entirely. In cauliflory, flowers burst straight from the woody trunk, and in flagelliflory whip-like branches trail from the trunk down to the ground, even below it, carrying the flowers. Hidden processes matter too. In barley, young flower spikes turn green while still wrapped inside leaf sheaths, and experiments that kept them in the dark found fewer flowers survived and pollen was less viable, especially near the spike's tip.
Source: Inflorescence
