I wouldn't go quite that far, but swelling is what they're doing, as this shot shows. Top growth has been disappointingly minimal, but it seems like they are concentrating their energies into developing their storage roots; these appear to be a combination of swollen hypocotyls as well as roots proper. Perhaps due to the limited rooting space, they're now pushing up, rather than down. I suppose this should be my cue that it's time to pot them on. I've avoided this so far, partially through laziness, but also because I've read that they resent root disturbance.
I was hoping for bindweed type growth to issue forth from the top of the plants like Silly String and wrap itself around any available object; what I've got so far is shown to the left. It could be, as I suspected, that it's not really warm enough here for that kind of exuberance. Or, another possibility - they're storing up energy for a more impressive performance next year. One can live in hope. In either case, it doesn't seem polite at this stage to take a bite out of any of them - not just yet. I am tempted, though. In the name of scientific enquiry, you understand.