Tomato stem primordia, sometimes called "adventitious roots". It's fairly common and often nothing to worry too much about. However, it can also be a sign of vascular or root disease/bad root conditions. If your plant is otherwise healthy and not wilting, yellowing, or dying, it's likely just a reaction to lots of rain or very humid atmospheric conditions. If you've been watering a lot, check soil moisture levels to make sure they're not too wet.

With such exuberant growth of root initials, can also indicate herbicide -- usually 2,4-D -- was used sometime in the past, and drifted onto the tomato.

Thank you. We have had very humid weather lately with very hot days and some erratic thunder storms.

Does that sound like a reasonable cause?

should I do anything at this point or leave it be?

m_gold: Thank you. We have had very humid weather lately with very hot days and some erratic thunder storms.

Does that sound like a reasonable cause?

That could definitely be a cause.

should I do anything at this point or leave it be?

There's not much you can do anyway. If it's caused by environmental conditions and you're growing outdoors, you can't control the weather. If it's caused by herbicide, you can't undo the damage. If it's caused by some kind of root or vascular disease, those diseases do not have cures.

Just keep an eye on the plants. If they continue to be healthy (aside from the usual fungal/bacterial foliage diseases that can happen this time of year but are unrelated to this issue), it's not a problem, just an eyesore. If they start to wilt and die, this means you need to get a disease ID so that you know what steps to take for next year.