Dr. Rebecca Shirk has studied genetic diversity and adaptation in native and invasive populations of Carolina cranesbill (Geranium carolinianum) during her PhD project in Department of Plant Biology at the University of Georgia in Athens. Flowers of this genus has a quite diverse pollination strategy.
Flowers come in a staggering diversity of forms. There are huge flowers; tiny flowers; flowers that never open; flowers with patterns that can’t be seen by the human eye; and extremely specialized flowers that are only visited by a single species of pollinator, to name just a few. Some plant species produce male and female flowers separately, either together on the same individual (monoecy; e.g. squash), or with separate male-only and female-only plants (dioecy; e.g. ginkgo). The reason for this great variety of shapes, sizes, and colors of flowers is reproduction: it is a plant’s strategy to make sure there will be seeds produced for the next generation. Colorful, showy flowers are usually meant to attract pollinators, such as insects or birds, for cross-pollination. But not all plants rely on pollinators, as many flowers have the ability to self-pollinate. “Selfing” can be advantageous because it provides reproductive assurance: even if there are no pollinators around, or if there are no other plants nearby to mate with, a selfing plant can still set seed. However, there are also drawbacks to selfing – notably, reduced fitness resulting from inbreeding depression. Self-pollination will arise only if the benefits (e.g., reproductive assurance) outweigh the costs (inbreeding). All of these hypotheses are encapsulated in Baker’s Law (1955).
Carolina cranesbill is a selfing species. It’s flowers are perfect – that is, they have both male and female parts. In this time-lapse video, you can watch how the self-pollination mechanism works. Starting from the center of the flower, you can see five light green knobs – these are the five lobes of the stigma, the receptive top of the ovary where pollination takes place. Moving out from the center, there are two whorls of five anthers each. The anthers are the male, pollen-bearing structures. For pollination and fertilization to occur, pollen must be transferred from the anthers to the stigma. As the video starts, the anthers look smooth because they are immature, but within the first few seconds, they turn nubbly as the anthers mature and shed their pollen. Then the anthers start to move, one whorl at a time, towards the stigma at the center. The stigma also opens up a little bit to help pick up pollen grains off of the mature anthers. After about thirty seconds (4 to 5 hours in real time), the flower has been successfully pollinated and starts to drop its petals. By evening, the petals will be gone and there will be five seeds developing at the base of flower.
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