YOUNGSTOWN — While adding to her knowledge of nature, Rosie Dempsey also seemed excited to add a new word to her vocabulary: photosynthesis.

“I learned about the green circles,” Rosie, 10, of Girard, said.

She was referring to having seen, under a microscope, dark splotches on a light-green leaf that, when magnified, resembled an artist’s rendition of a brick wall.

The dark areas contribute to photosynthesis, the process by which green plants and other organisms use sunlight and chlorophyll to synthesize food from oxygen and water while generating oxygen as a byproduct.

Rosie and younger brother, Ollie Dempsey, 6, had a hands-on opportunity to appreciate the role leaves play in the health of nature and the environment because they were among the children and adults who attended a 45-minute children’s gardening program and lecture Monday at St. Patrick Church, 1420 Oak Hill Ave., on the South Side.

Outside the church is a large garden in its beginning stages that will develop during the summer.

Also at the indoor gathering was Rosie and Ollie’s father, Nate Dempsey, who said the family has a garden next to their rear patio. They also grow blueberries and raspberries, he added.

Conducting the family-friendly discussion and demonstration, with a handful of green leaves next to her, was Stephanie Hughes of the Canfield-based Ohio State University Master Gardeners program.

“(Leaves) make sugar and oxygen and take out carbon dioxide that you don’t want in the atmosphere. They also preserve sunlight and water as catalysts,” Hughes explained.

Even though many people view covers of dead or decaying leaves in gardens as unsightly and an annoyance, they act as a warm insulator and blanket for bees and other valuable pollinators that nest in the ground in early spring, she said.

In addition, many varieties of leaves contain smooth, waxy cuticles on top that protect them, along with cells that hold moisture and oxygen when closed and release both when opened, Hughes said.

“In one way or another, everything eats something because of green leaves,” she said.

The program was held in the church largely because of the threat of rain and thunderstorms Monday afternoon.