So you start with a slightly wavy line. When you want to add a leaf, stitch an arc for one side, then the other, coming back up to where you started. Finally, continue the wavy line, creating the center vein of the leaf as you do.
Repeat this, staggering the placement of the leaves. Leave some space between the lines with leaves.
Now stitch a line adjacent to one of the leafy lines. Let the line have some wiggle to it. When you come to a leaf, let the new line echo around it. Do that again and again, letting each line respond to the lines next to it.
I have a ratio of about one leafy line to three echo lines, but you can of course play with that, or use only leafy lines and no echo lines for a different effect.
Here is what it looked like stitched up on some linen.