Haxnicks Root Trainers are so clever! The cells come flat, you snap them together and put them in the basket type holder, fill with soil and either plant seeds or take cuttings. Genius! You open it up when you're ready to plant. AND you can sneak a look at the roots without disturbing the plant! Again...genius! They are entirely reusable and last about 3 or 4 years so the $20 expense is really $5 per year. Amazing that somebody hadn't thought of this before...from the U.K. of course. Another English garden original.
"How to use them" videos are all over UK youtube.