Why do flowers have scents?
—H. JAMES, WOODBRIDGE, VA.

Natalia Dudareva, associate professor in the department of horticulture and landscape architecture at Purdue University, offers an answer:

Scent is a chemical signal that attracts pollinators to a particular flower in search of nectar or pollen, or both. The volatile organic compounds emitted play a prominent role in the localization and selection of blossoms by insects, especially moth-pollinated flowers, which are detected and visited at night. Species pollinated by bees and butterflies have sweet perfumes, whereas those pollinated by beetles have strong musty, spicy or fruity smells.

To date, little is known about how insects respond to the individual chemical components, but it is clear that they are capable of distinguishing among complex aroma mixtures. In addition to attracting insects and guiding them to food resources within the bloom, floral volatiles are essential for insects to discriminate among plant types and even among individual flowers of a single species. For example, closely related plant species that rely on different types of insects for pollination produce different odors, reflecting the olfactory sensitivities or preferences of the pollinators. By providing species-specific signals, the fragrances facilitate an insect's ability to learn particular food sources, thereby increasing its foraging efficiency. At the same time, successful pollen transfer (and thus sexual reproduction) is ensured, benefiting the plants.

Scent outputs tend to be at the highest levels only when the flowers are ready for pollination and when potential pollinators are active. Bees and butterflies tend to plants that maximize their output during the day, whereas flowers that release their fragrance mostly at night are visited by moths and bats. During development, recently opened and young buds, which are not ready to function as pollen donors, produce fewer odors and are less appealing to pollinators than older flowers are. Once a flower has been sufficiently fertilized, its bouquets are again reduced, encouraging insects to select other blossoms instead.

How are tattoos removed?
—T. DURKEE, BERKELEY, CALIF.

Dermatologist Joshua L. Fox, director of Advanced Dermatology's Center for Laser and Cosmetic Surgery in New York City, explains:

Industry experts say that 50 percent of people with tattoos will someday consider getting rid of their body art. Doctors remove the markings using three types of lasers: alexandrite, YAG and ruby. Each works on different pigment colors and compounds, so the dermatologist will use one or a combination of lasers depending on the nature of a given tattoo. (It follows that you would want to select a dermatologist who has the specific laser necessary for removing your tattoo.) Tattoo pigment is inserted into the dermal layer of the skin through ruptures in the top layer, or epidermis. To remove that pigment, the laser emits very short pulses, which are selectively absorbed by the color of the tattoo ink. This high energy fragments the pigment into smaller particles that are then removed by the body's immune system. In most cases, a series of laser treatments can remove 90 to 95 percent of the original design.

Patients who want a tattoo removed should seek a dermatologist with experience and equipment specific for the procedure. Good questions to ask include how many such procedures the practitioner has done and whether he or she owns the lasers or leases them. Doctors who own their lasers typically do more tattoo removals and as such have more practical experience.

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