The challenge was to identify mystery plant 15.

You received this hint in the puzzle's title:

"One, two, three and stop."

And this hint in the introduction to the puzzle:

"It's one, two, three when you count along with me."

And this hint in the caption of the photo that accompanied the puzzle:

"This is a clue."

...and these three clues.

When the pipers play Amazing Grace, tears well in my eyes.

When the bell tolls, it tolls for me.

When a flower blooms, my eyes rejoice.

The answer is piper's bellflower (also spelled pipers bellflower). You work it out by focusing on the third word in each sentence ("It's one, two, three and stop"; and "It's one, two, three when you count along with me..")

Thus.....

When the pipers play Amazing Grace, tears well in my eyes.

When the bell tolls, it tolls for me.

When a flower blooms, my eyes rejoice.

Perhaps you noticed that the photo accompanying the puzzle depicted pipers.

Named after Dr. Charles V. Piper, a pioneer Washington botanist (and early champion of soy bean cultivation), piper's bellflower (Campanula piperi) is an alpine wildflower that has been successfully cultivated in a few gardens but is endemic only in western Washington's Olympic Mountains. There it grows under harsh conditions on the drier slopes of the subalpine/alpine zones, typically in the cracks and crevices of rock outcrops but sometimes in scree.

People who see this flowering plant in the wild get bragging rights. You can see it for yourself in the high elevations of Olympic National Park. The Obstruction Point Road at Hurricane Ridge and the Deer Park Road (both of which are steep, narrow, and gravel-surfaced) provide access to the alpine zone, as do many of Olympic's wilderness trails.

This rare plant blooms in the summer, usually after snow melt in July and August. While the small blooms are typically blue (anything from dark blue or purplish blue to light sky-blue), a few plants produce pure white flowers.