Flowers are very useful for identifying plants and providing aesthetic pleasure for humans, but they have a more vital function—they are the sexual reproductive organs of plants. Many plants also have methods of asexual reproduction (vegetative reproduction), which produces offspring that are genetically identical to the parent: root-sprouting (limberbush, palo verde, aspen), stolons and rhizomes (agaves, strawberries, many grasses), and aerial plantlets (some agaves, mother-of-millions, kalanchoe). All of the progeny of asexual reproduction are clones of their parent plants. (A clone is a group of organisms that are genetically identical; in the case of flowering plants each clone originates from a single seed.) Horticulturists have developed additional methods of plant cloning that are valuable in perpetuating superior varieties of plants: cutting, grafting, and tissue culture. The ‘Kadota’ fig is a cultivar (contraction for cultivated variety) that has been propagated by cuttings for at least two millennia; it is described under a different name in the writings of Pliny the Younger.
In contrast, sexual reproduction combines half the genes from each of two parents, so sexually produced offspring are different from either of their parents and from one another. This variation is the raw material of natural selection which in turn results in evolution. A species that cannot reproduce sexually—there are quite a few among both plants and animals—is at greater risk of extinction if its environment changes, because it cannot adapt to new conditions.
Pollination is the transfer of pollen from an anther onto the stigma of a flower. The pollen then grows a tube (pollen tube) that penetrates the style down to the ovary; sperm cells swim down the tube and fertilize the ova. Fertilized ova develop into seeds, which are the sexual propagules of flowering plants.
Outcrossing (pollination by pollen from another plant) is evolutionarily advantageous because the offspring are more variable than those from self-pollination. Variability increases plants’ probability of surviving in an ever-changing environment. (But self-pollination is still sexual reproduction which results in different combinations of genes and therefore allows evolutionary change, as vegetative cloning does not.) Plants have many adaptations that increase the likelihood of outcrossing.
Pollen vectors in the desert
Because plants are rooted in the ground and can’t get together to mate, they must employ an agent to transport pollen between plants.
From this need widespread and complex kinds of mutualism (mutually beneficial interactions) have evolved between plants and animals. The pollen-transporting agent is frequently an insect or other flying animal. (Flying animals are more mobile than grounded species, and thus more likely to visit widely-separated plants.) In order to get pollinated, a flower must both make its presence known (advertise), and provide an incentive (a reward) for an animal to make repeated visits to flowers of the same species. The advertisements are fragrance and/or conspicuous color. Two kinds of food are the usual reward. Nectar is a sugar solution that provides energy for flight. Flying requires much more energy than terrestrial locomotion. Pollen, besides being the male gene-bearer of a flower, is also rich in proteins essential for maintaining animal tissues and for raising young. In place of nectar some flowers offer oil (fat), another energy food. Others provide fragrances that the pollinator gathers to use for its own reproductive advertisement, and a few fascinating species employ deceit and provide no reward (see the species account on pipevine for an example).
The sugar in nectar and the protein in pollen are expensive to produce, so there is selective pressure to use these resources efficiently. It is important that animals other than the pollinators do not eat (steal) the nectar and pollen, and that the pollinators transport pollen to other flowers of the same species and deposit it in the right place. Natural selection has produced specialization: most plants with animal-pollinated flowers attract only a few species of animals which have the right size and behavior to reach the reward and pick up pollen. The more than 100 million years of coevolution between flowering plants and their pollinators has greatly contributed to the huge number of species in both kingdoms (300,000 flowering plants, 350 hummingbirds, and 15,000 known bees in the world). It also explains why there are so many different shapes and colors of flowers.
Flowers can be classified into several pollination syndromes according to their pollinators. (A syndrome is a set of characteristics associated with a specific phenomenon.) This is not the same classification as systematic taxonomy and does not reflect the evolutionary relationships among plants. Species in the same family or even the same genus may attract different pollinators.
The hummingbird pollination syndrome is one of the most easily recognized. Hummingbirds are large compared to most insects, almost unique in their ability to feed while hovering, and daytime-active; they have no sense of smell, but have long narrow beaks and tongues that can probe deep narrow tubes, and excellent color vision. Hummingbird flowers tend to be long-tubular, non-fragrant, sideways- or downward-facing, day-blooming, and brightly colored. Bees and most other animals cannot easily land on a hanging flower, and even if they succeed they cannot reach the nectar at the base of the narrow tube.
There are common misconceptions that all hummingbird flowers are red and that hummingbirds can see only the warm colors of the spectrum. It is true that most hummingbird flowers in the temperate biomes are red, but in the tropics they come in many colors. The predominance of red in temperate hummingbird flowers may be a disincentive to bees. Bees are aggressive pollen collectors in temperate climates. But they cannot see red, so red flowers do not appear conspicuous to them.
Wind-pollinated plants make no investment in attracting animals; their flowers lack fragrance or showy parts. Many people would not recognize them as flowers at all. Prodigious quantities of pollen are released, an infinitesimal proportion of which lands on a receptive stigma of the same species. While this seems inefficient, it is obviously effective, judging from the successful groups of plants with this syndrome. Conifers, most riparian trees (such as willows and sycamores), oaks, and grasses are all wind-pollinated. Conifers and grasses are the dominant plants in the two biomes that bear their names. Grasses occur in most biomes and comprise the sixth largest family of plants with about 9000 species worldwide. Wind pollination is not always entirely passive.
- Plant Ecology of the Sonoran Desert Region, Arizona-Sonora Desert Museum.
- Pollination, The Ecotree.
- Phillips, S.J. and P.W. Comus (eds.) 2000. A Natural History of the Sonoran Desert. Arizona-Sonora Desert Museum Press, Tucson, and University of California Press, Berkeley.
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