February, 2000: French Guiana
Since the dawn of time, humans have utilized plants for their own purposes, including taking advantage of the attractant qualities of the scents produced by some flowers. Interestingly, those scents also perform a similar function in the lives of the plants themselves by attracting pollinators. Floral characteristics such as color, shape, and odor provide clues about the pollinator.
A moth-pollinated plant, which produces fragrance only at night.
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A hummingbird visiting a flower. (From Stern, Introductory Plant Biology, 8th ed., ©2000 McGraw-Hill Companies, Inc.)
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Abiotically pollinated plants are pollinated by nonliving factors, such as the wind or water. Usually odorless and dull in color, these plants generally lack petals and/or colorful sepals and do not produce nectar. Additionally, wind-pollinated plants are less efficient because they disperse large amounts of pollen into the wind in the event that the opposite sex of the same species is growing close by.
Flower markings on coneflowers (a) in ordinary light and (b) in ultraviolet light. (From Stern, Introductory Plant Biology, 8th ed., ©2000 McGraw-Hill Companies, Inc.)
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Biotically pollinated plants are pollinated by living organisms, including beetles, bats, flies, butterflies, birds, and the familiar bumblebee. Their flowers produce scents that attract these pollinators. The chemical composition of nectar, oil, and pollen rewards varies, depending on the pollinator--as does the scent produced. For instance, hawkmoth-pollinated flowers are very fragrant at night, but virtually scentless during the day when the hawkmoth is inactive. Bat-pollinated flowers are generally open only at night. These rhythmic changes in fragrance composition and nastic movements are the result of circadian cycles. The nectar of hummingbird-pollinated flowers has a high sucrose:hexose ratio. Most bird-pollinated flowers are red, and since birds do not have a keen sense of smell, the flowers do not waste metabolic energy producing scents. Bumblebee-pollinated flowers are typically blue or yellow with distinct visual markings, usually ultraviolet, commonly referred to as "bee markings." Evolutionary controversy surrounds the arrival of biotically and abiotically pollinated flowering plants. Some scientists hypothesize that abiotically pollinated angiosperms were the first flowering plants on the scene, while others believe that biotically pollinated flowers preceded them.
Floral scents can be produced by petals, stamens, staminoides, anthers, fruits, or even pollen. The scents produced are chemicals scientifically known as pheromones, a group of volatile, organic compounds, first identified in insects, that function as chemical communication signals and sexual attractants. It has been proposed that humans too produce pheromones, since steroidal hormones such as androsterones have a noticeable odor. Humans also have glands at the base of hair follicles that produce chemicals with a detectable odor--chemicals for whom scientists have been able to identify no other purpose beyond serving as attractants.
Since the dawn of time, humans have been able to select and use flowers to create specific scents in the home and for the body. Ancient Egyptians used scented wax formed into columns worn on top of wigs on their heads; as the day progressed, the wax slowly melted, releasing the pleasant aroma. Boswellia and Commiphora, more commonly known as frankincense and myrrh respectively, produce a fragrant resin that is often used as incense.
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Osmophores (X100). (SEM © 1999 Erica Kipp.)
C. guianensis of the Lecythidaceae family. (Photo © Amy Litt.)
Today, fresh plant material undergoes several different extraction methods to yield concentrated oil. For sale, the oil is diluted in an alcohol, resulting in perfume, eau de toilette, or cologne depending on the percentage of oil in the final product. Many popular flower scents come from the entire surface of the petals, where scent glands called osmophores are found. The secretions of this tissue are invisible and highly volatile, so they must be captured within a specially designed glass bowl. The bowl is in two separate halves that fit around the flower and capture its scent on special adsorbent material called Tenax. The Tenax is then brought back to the laboratory for analysis such as mass spectrometry and gas chromatography (GCMS).
Using scent as aphrodisiacs and attractants is a big business for Givaudan Roure, an international perfume house that is responsible for the scents in such products as Bijan's Michael Jordan cologne and Herbal Essence shampoos and conditioners. A fragrance research scientist from Givaudan Roure recently accompanied Dr. Scott Mori, director of the Institute of Systematic Botany at the New York Botanical Garden, to French Guiana in search of the next aromatic elixir. There, Givaudan Roure scientists used glass bowls to capture the secretions from flowers in the Lecythidaceae family. This family of dicots is commonly known as the Brazil nut family, and its members are found mainly in tropical South America. Givaudan Roure brought back many samples from Lecythidaceae, including Couroupita guianensis and Gustavia augusta. C. guianensis has been described as the sweetest smelling flower in the whole Lecythidaceae family. It may be the next Chanel No. 5.
References, Websites, and Further Reading
Endress, Peter K. 1994. Diversity and evolutionary biology of tropical flowers. New York: Cambridge University Press.
On the trail of a scent. Natural History, The Flower Issue, American Museum of Natural History (May 1999): 68-72.
Information on Givaudan Roure, a division of Roche Holding Ltd. of Switzerland
Information on Lecythidaceae, including links into pollination and dispersal biology
Pollination biology of plants from Central French Guiana
Stern, Introductory Plant Biology, 8th Edition
Chapter 1: What Is Plant Biology?
Human and animal dependence on plants, pp. 4-7
Chapter 2: The Nature of Life
Carbohydrates (e.g., sucrose), pp. 22-23
Lipids (e.g., oil), p. 23
Proteins, pp. 24-26
Chapter 4: Tissues
Secretory cells and tissue in the secretion of nectar, oils, etc., p. 60
Chapter 8: Flowers, Fruits, and Seeds
Monocots and dicots compared, pp. 129-30
Chapter 19: Kingdom Fungi and Lichens
Pheromones, p. 338
Chapter 23: Flowering Plants
Pollination, fertilization, and seed development, pp. 421-23
Pollination ecology, including UV bee marking, hummingbird- and bat- pollinated flowers, pp. 425-30
Appendix 2: Biological Controls
Pheromones, pp. 508-9
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