Once Clair Russell Ossian becomes interested in a subject, he is compelled by some inner drive to become an expert.

A geologist often invited on archaeology digs in Egypt, Mr. Ossian noticed repeated images of water lilies carved on ancient relics. "They're the most commonly seen artistic element in Egyptian art," he says. To understand why, he felt he had to grow them. To grow them, he needed a pond.

Once he had a pond, he wanted to add koi to it. But koi demand high standards of water purity. And they are notorious for uprooting and eating aquatic plants. Furthermore, the professor notes in yet another aside, koi like to swim up and down as well as round and round.

The third version of his backyard pond, therefore, takes into account all he has learned. There is a lower pond for the water lilies and other aquatic plants, safe from koi nibbling. The upper pond shelters the brightly figured carp in water treated via a multistep biological filter and zapped by ultraviolet light.

To keep the koi entertained, Mr. Ossian varied the upper pond's depth from shallow to 4 ½ feet deep. This gives the fish, some 3 feet long and including a black koi for good luck, variety in their daily routine.

With his endless supply of little-known facts and interesting anecdotes, Mr. Ossian will mesmerize visitors to the North Texas Water Garden Society's annual pond tour this weekend. His Carrollton water garden is one of 46 stops on a route that includes six North Texas counties. Ticket holders can visit any number of ponds on the self-guided tour, including several that will be open for evening visits.

"Ponds look totally different at night than they do in daytime," says Joe Copeland, president of the NTWGS. The sound of moving water is magnified at night, backgrounds fade into darkness and specialty lighting alters visual perceptions.

The tour is scheduled in the summer to give tropical plants, especially water lilies, time to bloom. Some of the gardens open for evening visits feature fragrant, night-blooming plants.

Mr. Ossian's rare Egyptian water lilies (Nymphaea caerulea), however, are day bloomers. Sky blue and fragrant, they were an important symbol in that civilization's art and culture. The geologist, who is president of the North Texas chapter of the American Research Center in Egypt, says the flower is depicted on clothing, held out as an offering and worn by women on their headdresses. Their likenesses are on the sarcophagi of pharaohs, laid square on their foreheads or placed like a wreath around the carved figure's neck.

The plant is not showy by today's standards, but it is used as one of the primary parents for modern day-blooming tropical hybrids. The starburst bloom is small, about 6 inches across. The life of the flower is three days, opening in midmorning as the sun rises and closing in midafternoon.

One of the reasons for the flower's sacred status may be its perfume, explains Mr. Ossian, who won't reveal how he acquired his lilies from Egypt. He reaches into the pond to pluck a just-opened blossom and holds it out to be sniffed.

Its perfume defies an exact description. There's a hint of lily of the valley; or maybe gardenia, but not as intense. Still, there is a top note in the ethereal, complex fragrance that eludes identification.

The Egyptians believed that "if you suddenly smelled a wonderful scent in the air, it meant a god is near," says Mr. Ossian. He has been preserving dried water lily flowers for four years, and the petals still retain a sweet fragrance.

The professor, who teaches at the Hurst campus of Tarrant County College, also raises lotus, which throws one or two huge pink flowers, and papyrus in his water garden. "Being a geologist, I love ancient plants. Anything that comes into our house has to be invasive and oversized."

He also tends collections of begonias, orchids, carnivorous plants and succulents, all grown in pots. Come winter he erects a pop-up greenhouse for them, warmed by a space heater. Another pop-up greenhouse goes over a family of cycads (parents and pups) planted in the ground. Resembling trunkless palms, the immense fronds grow in a protected spot against the back of the brick house. The specimen Mr. Ossian and his wife, Eleanor, call Big Mama is developing a cone, which, if pollinated, bears the dinosaur-era plant's seeds.

"The world is such a wonderfully interesting place," he observes.Tour notes

Pond tour booklets ($10), which include color photos, highlights and addresses of all the stops, are for sale at 14 area retailers. Go to www.ntwgs.org to see the store list. Some ponds will be open both Saturday and Sunday; some will be open until midnight Saturday; and others will continue to be accessible by special arrangement through September.