Plant of the Month: The four o’clocks

Mar 31, 17 • 5enses, Plant of the MonthNo Comments

By David Moll

Nowadays, classification of organisms is definitively established through genetic analysis. Before this technology, there were many morphological and biochemical traits used to sort organisms into related groups. Within the flowering plants, it was the flowers themselves that were used as a pivotal characteristic. The number and arrangement of floral parts were like a beacon leading botanists through an overwhelming forest of malleable growth forms and secondary attributes.

The four o’clocks provide a brief but interesting floral stumbling block. That large, often colorful, structure that seems to be a corolla, is actually a calyx formed of united sepals. Commonly aiding in this illusion is a united group of bracts below the flowers that resembles a more typical calyx. Contrary to outer appearances, these attractive plants have no flower petals.

Flower parts come and go throughout evolutionary time, but botanists can tell the difference between these various structures by examining vascular traces in the flower. Once this was sorted out, the four o’clock family (Nyctaginaceae, which includes Bougainvillea), unlike many others, has so far remained intact through all the technological scrutiny. We don’t need to be so technical. Once learned, mostly in the field, unifying characteristics of plant families can lead us to identifications, understandings, and greater appreciation.

Don’t be fooled. Even though it has no flower petals, the Colorado Four O’clock is a showy favorite in the Prescott area. Courtesy photo.

The four o’clock name may also be misleading. It comes from the habit of flowers opening in the late afternoon and closing the following day. The exact timing is greatly influenced by weather conditions, however.

More four o’clock deception occurs with the Trailing Four O’clock. It has three flowers pushed so tightly together so as to appear to be one flower.

Colorful flowers indicate animal pollination as opposed to wind pollination; nighttime blooming suggests moth pollination, but all conditions can be pollinated by several vectors. The Sweet Four O’clock has a white flower (which stands out in the dark) with a magenta center. It also has a four-inch, narrow tube that leads to nocturnal nectar, and is pollinated — though not exclusively — by hawkmoths that have a long tongue to match that floral tube.

So things are not always as they appear in the world of flowering plants. It’s a terrific time of year to get outside and examine plants for their sometimes obscure, sometimes beguiling flowers that provide an opening into the history and ecology of Earth.


Find out more about the Arizona Native Plant Society at AZNPS.Com.

David Moll studies nature in Arizona.

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