One common type of inflorescence, is one in which flowers open up one or a few at a time, for an extended period. Examples include lupines, snapdragons, gladioli, and foxgloves. Such inflorescences are adapted to induce "repeat visitors" - insects, birds, or other animals that remember the location of the plants and drop by each day to collect nectar or pollen from freshly opened flowers. This is a behavior known as trap lining.
In such flowers, a common means of avoiding self-pollination, is for the stamens and pistil within each flower to mature on different days. For example, pistils may be active and receptive to pollen on the day the flower opens, with the anthers opening to shed pollen 24 hours later. The common Amaryllis follows this pattern.
|A common means of avoiding self-fertilization within individual flowers is to have pollen shed on one day (left) and stigmas receptive on another day (right). This effectively makes the flowers male on one day, and female on another day.
Compound, or false flowers, such as those of the sunflower family or the spectacular poinsettias, are actually condensed inflorescences adapted to look like a single large flower to pollinators, but still opening their flowers a few at a time to attract trap-lining animals.
|The Titan Arum, Amorphophallus titanum,
blooming at the U.S. Botanical Garden in
Washington, D.C., posted on Wikipedia.
The Titan Arum, which makes the news whenever it blooms in a botanical garden, produces a gigantic inflorescence in which all the tiny flower buds are mature when the large bract, or spathe, opens to reveal them. They follow a similar strategy as the amaryllis flowers above, with separate female and male phases. In this case, all the female flowers are active first, followed by the male flowers in a day or two. This avoids self-pollination, as insects arrive during the female phase, bearing pollen from another inflorescence. They then leave during the male phase, freshly dusted with new pollen.
After waiting for a few hours at the edge of a swamp in Costa Rica, I found that inflorescences of a species of Bactris opened abruptly at dusk, displaying unopened male flowers and active female flowers nestled within them. The inflorescence emitted a musky odor, which attracted a variety of small flies, bees, and beetles. The male flowers opened to release their pollen 24 hours later. I observed the same thing in another species of Bactris later.
A year later, I was in Papua New Guinea and observed a nearly identical process in a species of Hydriastele. I was able to get more detailed pictures of male and female flowers, along with their insect visitors, which I share below.
|Though it consists of a number of branches, the inflorescences
of Bactris guineensis behave the same as that of the Titan Arum. Female
flowers, hidden among the larger male flowers, are
receptive to pollen soon after the large, fibrous bract opens.
|When the bract of a Hydriastele microspadix inflorescence splits
open, the flowers are all mature, and arranged in triads of
two large male flowers with a tiny female flower between them.
|Female flowers of Nypa form a dense globose head (left), and appear to provide no nutrition for insects. The dense male spikes (right), however, provide a place for fly larvae to develop as they feed on the tissues of the unopened male flowers.