Local filmmaker Michael Schwarz  spent many years wondering if he’d ever find a way to make the documentary “The Botany of Desire.”

Potential donors were wary about one of the four plants — marijuana — featured in the film, and he was repeatedly turned down for funding.

Worried that including marijuana in the film would promote drug use, potential donors again and again said no.

Finally, Schwarz received one small, no-strings-attached grant in 2005 — a turning point that allowed him to start making the film, which is based on Michael Pollan’s 2001 bestseller of the same name.

“The Botany of Desire” airs 8 p.m. Wednesday, Oct. 28 on KQED.

During the time he was struggling to find funding, Schwarz decided a good strategy would be to tackle the subject that had been his biggest roadblock: marijuana.

So he went to Israel.

There, Schwarz found the man who transformed brain research with his discovery of THC, the particular chemical in marijuana that affects human brains.

“The Botany of Desire” traces the apple’s journey from its origin in Central Asia to America.

Raphael Mechoulam, a professor at Hebrew University, was the first person to identify and isolate THC in 1964 at the Weizmann Institute of Science in Rehovot, and that opened the door for groundbreaking neuroscience.

“His discovery is amazing,” Schwarz said. “And until we could show people that story, we were fighting an uphill battle.”

After completing and screening the 30-minute segment on marijuana, Schwarz found full funding. The result is a two-hour documentary about the human relationship to four plants — marijuana, apples, potatoes and tulips — narrated by Academy Award–winning actress Frances McDormand.

The film examines the history of each plant and how each evolved to satisfy humankind’s most basic yearnings — for sweetness (apple), beauty (tulip), intoxication (marijuana) and control (potato).

“We don’t stand apart from nature. We stand in nature,” Schwarz said. “We’re part of a larger web. Nature is acting on us as we act on it. Our fates are tied together.”

Schwarz lives in Menlo Park with his wife and two children. He grew up in Woodmere, N.Y., where his family attended a Reform synagogue, and said he has strong Jewish values “that have informed everything I’ve done and everything I am.”

Schwarz first met Pollan in the late ’70s, when the Jewish men both worked as magazine editors in New York. They remained friends, and in 2000, when Pollan finished the draft of “Botany,” he sent the manuscript to Schwarz, who immediately envisioned a documentary.

The film traces the apple’s journey from its origin in the ancient forests of Central Asia, across the Silk Road to Europe, and eventually to America; the tulip’s rise to fame in 17th-century Holland; the potato’s trek from the Andes Mountains to Ireland to the fields of Idaho; and marijuana’s journey from the Far East to Mexico to California greenhouses.

Pollan, now a U.C. Berkeley professor, is interviewed throughout the film, along with farmers, tulip breeders, scientists, professors and authors. And one of the primary characters is Mechoulam.

The Israeli biochemist “told us some very funny stories about experimenting with his own friends, including members of the Knesset, by baking brownies” that contained various isolated chemicals found in marijuana, Schwarz said.

“Through these homegrown trials and errors, he figured out that THC is the molecule that somehow changes our consciousness,” he added.

PBS has created educational materials that teachers can use in conjunction with screening the film in schools, although Schwarz knows some teachers might shy away from showing the marijuana section.

“One of the points the film makes is that the desire for intoxication is hardwired in us, and you see it even in young kids spinning around or playing on swings,” he said. “Kids are going to experiment, so it seems to me that it’s valuable for them to understand what it is about their brains making them seek an altered state of consciousness, and to understand the consequences.”

“The Botany of Desire” airs 8 p.m. Wednesday, Oct. 28, on KQED–Channel 9. For lesson plans and more information, visit www.pbs.org/thebotanyofdesire.