In the Madagascan countryside people who need medication consult an ombiasy, or healer, with a special connection to plants and knowledge of sacred artifacts used to invoke their healing power. We visited the home of one such healer, four small rooms with dirt floors. Seated on wooden benches in the waiting area, we watched patient after patient leave with hands full of plants—one with the greenish Jteorhiza palmate vine to deaden pain, another with the leaves and bark of Harungana madagascariensis, which stimulate gastric juice secretion, useful in the treatment of digestive disorders. Scientists have documented the effects of both these plants.

I asked the healer where her medicine came from, expecting her to say that a lack of money prevented her from buying all the necessary medical plants. Instead she waved her hand in a sweeping motion. Quansah explained: "She just walks out of her house and picks whatever plants she needs."

How much longer will she be able to do that, I wondered as we continued our drive across the island. Along the road that connects east and west Madagascar—potholed and in some places washed away—we encountered mile after mile of broken, smoldering trees, where slash-and-burn farming, fueled by population pressures, has left so much of the land bare. Streams flowed dark red with soil runoff. Of Madagascar's thousands of plant species, Quansah said, a great number are in danger of extinction. "What once was a rain forest is now a rain desert," one local man told me. "This place was very beautiful when I was a child. It was a forest filled with lemurs."

"Stop the car!" I shouted. Thinking of all the potential cures for cancer and other diseases being lost, I traced an "X" in the middle of the road and asked Quansah to stand on it. Not hiding that I was upset, I asked him to stand there until he could see a medicinal plant that grows only in Madagascar.

He examined the bushes and scrub trees on either side of the road and pointed to a vine hanging 20 feet away: "that purple flower there."

As we walked toward it, he began to show me other medicinal plants we passed along the-way. We hadn't left the road when he stopped and touched a tall leafless stem that, crushed and brewed into a tea, acts as an anesthetic. Chemicals in the leaves of another bush, he said, kill a certain virus. More than an hour later we still hadn't reached the purple flowers. Every step of the way, Quansah showed me more medicinal plants.

In Romeo and Juliet William Shakespeare describes "the powerful grace that lies in herbs." It was becoming clear to me that plants' powerful arsenal of bioactive substances—compounds that affect living cells—can be of significant value in waging war against human ailments.