This captivating and true story recounts the adventures of three scientists in the Amazon region: the father of modern ethnobotany, Richard Evans Schultes, and two of his former students, Timothy Plowman and Wade Davis, the author of this book. It reveals that scientific adventure is not just a thing of past centuries but has continued well into the 20th century.

This is not strictly a biography of Schultes since it deals mainly with the period of his most extensive field work in tropical America, starting in Mexico in 1936 and continuing almost without a break in western Amazonia until 1953. The biographical chapters about Schultes are interspersed with details of the expeditions of Davis, Plowman and their faithful dog Pogo over a 15-month period in 1974-75. Theirs was partly a pilgrimage in the footsteps of their mentor through Colombia, Ecuador, Peru and Bolivia, and partly a pursuit of new discoveries: the blending of two experiences more than two decades apart works extremely well.

Schultes is a remarkable man who on the one hand is more conservative than Margaret Thatcher and a Harvard professor who hardly acknowledges America west of the Hudson river, yet on the other is the world's leading expert on hallucinogenic plants, who sparked the psychedelic era of the 1960s with his discoveries. He also, while posted to the Amazon during the second world war, became the foremost expert on the taxonomy and the variation of rubber, while searching for high-yielding varieties to replace the supplies cut off from the Far East by enemy action.

He spent a great deal of time with indigenous peoples. One reads of a man who related extremely well and sympathetically to them and consequently discovered a huge amount of new ethnobotanical information. He described the Kof n Indian leaders as "friendly, helpful, intelligent, trustworthy and dedicated". They probably had a similar opinion of him. As a result Schultes also became an authority on medicinal plants, about which he has published several books and numerous articles.

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Plowman, following in the Schultes tradition, became the world's leading authority on the coca plant, the source of cocaine, and demonstrated that, as used by South American natives, it is much more than a stimulant, being a vital source of nutrition in their diet. It is good to have a perspective of Plowman in the field because he died of Aids at the untimely age of 45. In fact it was his memorial service, at which Davis delivered the eulogy and Schultes spoke via a taped message, that inspired Davis to write this book.

Schultes's fascination with plant hallucinogens began as an undergraduate in 1936 on a Harvard-Yale-American Museum expedition to Oklahoma to study the use of the cactus peyote as a hallucinogen by the Kiowa people. How could a young student not have become interested in ethnobotany as a career, after his experiences with Charlie Charcoal, the nephew of Kicking Bear and Mary Buffalo? It was there that Schultes began sampling the products he studied. He never understood why on that occasion he only saw bright colours and did not experience true hallucinations. His second expedition in 1938 was to Mexico in search of the magic mushrooms called teonancatl or "the flesh of the Gods" which he successfully tracked down, sampled, and reported upon in the Botanical Museum Leaflets of Harvard University. After a third expedition, again to Mexico, he returned to Harvard and completed his doctoral degree. A measure of the man is that when he left for South America on a Guggenheim fellowship to study arrow poisons in northwest Amazonia, aged 26, he had left in press two books and scientific papers.

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By the time that the Japanese bombed Hawaii, Schultes was ensconced at Mocoa in the Colombian Amazon studying not only arrow poisons, and a new species of fish poison which he later named Serjania piscetorum, but also hallucinogens. He sampled yage made from the bark of Banisteriopsis caapi. His visions were blue and purple in slow undulating waves of colour. However, when he sampled yage mixed with chagropanga he experienced "reds and golds dazzling in diamonds that turned like dancers on the tips of distant highways".

Schultes's own hero is the 19th-century British botanist Richard Spruce (1817-93) who travelled the length of the Amazon between 1850 and 1856. Spruce's diaries recount graphically the deprivations and illnesses which he suffered, yet he survived, made the most important collection of Amazonian plants of his century, and also made many observations, including ones about hallucinogenic plants that were only confirmed a century later by Schultes. Davis recounts how Schultes experienced many difficulties similar to those of his hero, such as extensive ulcers on his legs, blood poisoning, and several serious bouts of malaria. He too was a survivor so dedicated to his mission that he was never deterred by the inevitable problems of conducting fieldwork in remote places. Once, when his outboard motor failed, Schultes, with minimal supplies of food, drifted down the Apaporis river, portaging the famous Jirijirimo rapids and many others until his party reached a mission station where he could communicate with Bogot . In the meantime his boss at the rubber exploration unit had declared him missing and presumed lost. Even during that adventure he collected interesting plants, by exploring while his crew rested during the frequent breaks which he ordered.

The Plowman-Davis expedition rivalled that of their professor in both adventures and successes. Their part of the book recounts a journey of many thousands of miles in a red Dodge pickup truck, during which they collected over 10,000 dried plant specimens and hundreds of living collections in the form of rhizomes, cuttings and seeds. They proved to be worthy disciples of their professor, equal in their enthusiasm and their curiosity to experience the mysteries of the many hallucinogenic plants that they collected. Towards the end of the book Davis recounts his own experience of taking yage. "At first it was pleasant, a wondrous sense of life and warmth enveloping all things. But then sensations intensified, became charged with a strange current . . . The beauty of colours, the endless patterns of orblike brilliance were as rain falling away from my skin . . ."

But this fascinating book is much more than the adventures of three botanists and their encounters and experiences with hallucinogenic plants. It is a deeply researched account that brings together history, biographical insights into many well-known botanists such as Hernando Garcia-Barriga and Jose Cuatrecasas, and some fine descriptions of the rainforests and their natural history. Howler monkeys, iridescent butterflies, wallowing tapirs and spectacular displays of toucans are all involved. The tragedy is that both the forests and the peoples described here are fast disappearing. The work carried out by Schultes, Plowman and Davis becomes all the more important when one considers how much of this body of knowledge and human experience is being lost through the pressures of western development in the Amazon region. Davis is to be congratulated on a thorough piece of research that will give future generations a glimpse of the extraordinarily varied plant lore of the South American natives and of the courage of those who have studied it.

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Sir Ghillean Prance is director, Royal Botanic Gardens, Kew; he has conducted 15 expeditions to study Amazon flora.