I found this stitch in a vintage (1964) Coats & Clarks booklet called "One Hundred Embroidery Stitches"—the illustration is reproduced from that book. Called overcast stitch (or trailing) stitch it's kind of a cross between couching and satin stitch. It resembles a fine cord and would be perfect for stems instead of outline stitch. Here's how to do it:
Start by bringing what we'll call the laid threads up to the surface. I used six strands of floss because I wanted a nice thick cord; use less for a more delicate one. Rethread your needle with a second thread in the same color (I used three strands) and bring it to the front using the same hole as the laid threads.
Hold the laid threads in position on the line of your design with your left thumb.
Work small satin stitches very closely together over the laid threads. You're doing the same technique as with couching but with that stitch they're spaced farther apart.
When you come to the end of your stem, rethread the laid threads onto your needle and take it to the back. Then take your overthread to the back in the same hole. If you're working a short line it might save time and be less hassle to use two needles and keep the first threaded with the laid threads. Either way is fine.
Here's what it looks like when you've finished. My line is kind of wavy—I think I need to practice keeping it straighter :)