I've grown shiso from seed many times and feel (felt?) like I knew their growth pattern, and how to prune them to make for bushy plants. I pretty much grow it like basil. But I have one seedling that started normal with all the others and then went berserk, like this:

No complaints as its just a mass of new leaves and all seem to be growing but I was so curious as to what it's doing. I would have thought it's bolting, but 1. it's not hot enough yet and 2. after having watched it grow out, there are only new and normal leaves -- no sign of flowering.
Here's a closeup of the backside of this celery like growth.

Any ideas? Maybe this is just a different cultivar of shiso than I had in the past and this will end up being a silly question, but none of the other plants, all started from seed on the same day, are doing this.

Grace

The only odd thing that all the shiso in this pot experienced was what looked to be a lack of calcium absorption early on. It affected them up until the start of the second set of leaves when I moved them out of that pot and into this one where they have now been for 5 weeks or so. All the growth since then softened up and became normal. I pruned the tops of all others in this pot (which then resulted in leaking stems for a week!?!) except this one, where I wasn't sure what it was doing or where/how to chop.

This same plant is growing one ridiculously large shoot/stem, enough to make me think a seed had inadvertently been planted there, except it is connected to this plant, I found, once I dug in a little.

Fascinating stuff! Thank you for identifying this!

Grace

This fasciation business is so cool. Here's a shot of a plant from the same seed planting. Only one turned into this beast of a plant; the rest are as expected and normal-sized....and slowly growing.