Inflorescences#

A Plumeria inflorescence is a grouping or cluster of flowers, as opposed to solitary flowers borne separately. Sometimes the word floret is used to refer to the individual flowers within such a cluster.

Inflorescences can be:

• • spike — perfect (bisexual) flowers growing on the side of a single stalk, attached in a sessile manner
    • peduncle is the base of the stalk, attaching the inflorescence to a stem
    • rachis is the main spine of the stalk above the first, oldest flower
    • Verbena lasiostachys or common verbena
  • raceme — similar to a spike, except the individual florets are attached to the rachis with a distinct pedicel or “stemlet”:
    • Sometimes these are more or less straight: Penstemon centranthifolius or scarlet bugler
    • Other times they are scorpioid, with the tip curling like a scorpion’s tail: Amsinckia menziesii or common fiddleneck
  • catkin — also similar to a spike, except the florets are generally imperfect (unisexual). They also usually lack petals or sepals, probably because they are wind-pollinated and don’t need to “tart up” to attract a pollinator. They often have small bracts, or leaflike/sometimes almost scaly structures, at their bases. They can be sessile or attached to the rachis with a very small, easy to miss pedicel. The inflorescence can be upright, but more usually it droops downward from a leaf axil, often looking like a fuzzy caterpillar, what with all the stamens or pistils sticking out.
    • upright: Salix exigua or narrow-leaf willow
    • drooping: Quercusa agrifolia or coastal live oak
  • spadix is another variant on the spike theme. Imperfect flowers grow along the spadix, but both sexes can be found on the same spadix, but the genders are separated, with female flowers toward the bottom and male flowers toward the top. The spadix is surrounded by a drastically modified bract, called a spathe, creating a calla lily-like impression. The spathe can be vividly colored. There are none of these native to Southern California, but there there is a Northern California species of interest: skunk cabbage
  • panicle — a more elaborate raceme, with two or more florets, each on pedicels, attached to a rachis, which itself may be attached to a main axis coming out of the peduncle: Rhus integrifolia or lemonadeberry
  • corymb — similar to a raceme (simple corymb) or a panicle (compound corymb), except the lower pedicels are longer than the upper ones, so the whole inflorescence looks flattened toward the top: Achillea millefolium or common yarrow
  • umbel — pedicels all radiate directly from the end of the peduncle, without a rachis in the middle.
    • Sometimes, the pedicels radiate out more or less in the same plane, creating a flat-topped inflorescence that might be confused with a corymb: Daucus pusillus or American wild carrot
    • Commonly, they radiate in all directions, forming a ball-like inflorescence: Aralia californica or elk’s clover indeterminate: This means that the cluster or stalk of florets grows out from the outside/upper part of the inflorescence, so the youngest ones are up on the very tip, which keeps on growing out. Rhetorically, this could go on “forever,” with no pre-determined end to the process, hence, “indeterminate.”
• determinate: This means that the cluster or stalk has terminal florets, which are the oldest ones in the inflorescence. They cap further development of the peduncle or supporting stalk. Younger florets emerge from the side of the peduncle or the center of the cluster, which pretty much puts a limit on how long this can go on (the fixed-length peduncle can only accommodate so many new florets, hence, determinate.
  • cyme is a branched inflorescence with as few as three florets arranged with two on either side of a rachis sporting a terminal floret. It can also be pretty elaborate, with multiply branching units. What they all have in common is that the central florets mature first and the peripheral ones later: Phacelia ramosissima or branching phacelia is an example, and California wild roses, Rosa californica, sometimes develop cymes, mixed in with solitary flowers.
• category-busters:
  • thyrse is a raceme (indeterminate structure), which has cymes (determinate clusters) instead of individual florets. An example of this is the complex inflorescence of the true sages, which put forth tall stalk-shaped inflorescences along which ball-like cymes appear at intervals: Salvia mellifera or black sage
  • composite head looks like a single flower but is a combination of numerous individual flowers. Petal-like ligules are actually individual ray flowers, which may surround a collection of tiny circular disk flowers.
    • radiate heads have both ray and disk flowers: Encelia californica or California encelia
    • ligulate heads have only ray flowers: Taraxacum californicum or California dandelion
    • discoid heads have only disk flowers: Hazardia squarrosa or common hazardia
  • solitary flowers — not all flowers are inflorescences. Many are solitary flowers. These can be:
    • terminal found at the end of a stem: Romneya coulteri or matilija poppy
    • axillary with the peduncle emerging out of the axil between a leaf and a stem: Cucurbita fœtidissima or calabazilla

There are other structures associated with inflorescences:

• bracts or leaf-like structures at the base of an inflorescence or at the base of an individual floret’s pedicel. They generally look different in some way from regular foliate leaves and from sepals. Usually, they’re green, but they can be loudly colored so that you think they’re a petal (e.g., garden bougainvilleas, the “spathes” of calla lilies)
• involucre is a whorl of bracts under (or “subtending”) an inflorescence.
• In composite flowers, the involucre under the flower head looks like the calyx under a normal flower. Instead of a calyx’s sepals, the composite involucre is composed of bracts, termed phyllaries.

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