(Photograph = Opuntia macrocentra, Matt Baker)
Introduction
How many plants must be studied in an experiment? How many plants are needed to determine a fact that can be applied to the whole species? Does a study of 10 plants give enough information about Opuntia macrocentra to know all about it? Does a study of all the plants in a plot that is 20 ft x 50 feet provide enough information? How many plots are needed to make a broad conclusion about the species?
The Details
Scientists wanted to study the biology of O. macrocentra flowers. They marked out five plots (20 ft x 50 ft) in Radium Springs, New Mexico at the Chihuahuan Desert Rangeland Research Center in New Mexico. They studied all the plants in the five plots for two years.
Plants of O. macrocentra first make flowers when they have about six cladodes or more. Fruit production varies from zero to 30 fruits per plant. The fruit/flower ratio was measured as well as the amount of nectar that flowers produced. Additionally, time of flower opening and flower size was recorded each day.
Certain experiments were conducted. Flowers were allowed to be pollinated by natural means; flowers were hand pollinated with pollen from other flowers; and flowers were self-pollinated. Also, fruits were collected and dried and the number of seeds in each fruit was determined.
Flowers were open only during the day, for about 9 to 11 hours. Peak nectar production was in the late morning and early afternoon. Cross pollination caused more seeds to be made per fruit than self-pollination. Many flowers were damaged or destroyed by a caterpillar (Olycella subumbrella). Flowers were mostly visited by solitary bees (non-hive bees) that pollinated them.
These results seem likely to be representative of the entire species. But, O. macrocentra is found from Arizona to Texas and into Mexico. It seems possible that some populations could produce more nectar or less nectar. Other populations might have larger flowers, and so on. But, this study seems to provide a good starting point for understanding the flowers and fruiting of the species.
Additional Reading: Reproductive Ecology of Opuntia macrocentra
