Cable
Omono
If you get big callus and no roots, then there is a bridge somewhere. You have to look hard and feel, sometimes the bridge will feel slimy. Whats really weird is that the bridge needs only to be the width of a toothpick and it will keep the top from issuing roots.
Wait, time-out. Is that always the case or can it just fail if you have a species that roots crappy?
This is what I got when I tried to air-layer a grafted tamukeyama:
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I didn't notice a bridge anywhere, though. I haven't thrown it away yet so I can double check tomorrow.