It's a spontaneous mutation, where a plant suddenly gets sick and tired of having "round" stems, and decides to try out being "flat" for a while.
Forsythia does it a lot:
This example shows a flat, fasciated stem, and the fact that there are shoots and leaves coming out of it, also shows that the plant continues to grow in an otherwise normal way.
I've mostly seen this mutation in shrubs, with their woody stems, but I've also seen it in herbaceous plants: I could've sworn I had pictures of a fasciated weed which I found locally, a few years back, but I can't find it. This is a lesson to Always Label Your Photos.... anyway, back to the fasciation.
Summer Jasmine is a little tinker for showing this mutation: here's a really good example, right: on top is the super-fasciated stem, and below it is a normal stem, for comparison.
It doesn't seem to affect the plant, other than cosmetically: it continues to grow, although sometimes fasciated stems get quite contorted, so they can spoil the pleasing outline of a plant.
Today I found a new addition to my list of Plants Which Fasciate: and that's Hibiscus.
I've not seen them do it before, but today, whee hee! there it was, a beautifully flattened stem. Luckily it was on a part of the shrub which was being pruned anyway, so the owner didn't mind me bringing it home and taking photos of it.
It almost looks as though there are many stems welded together, but that's not the case, it's just one stem which has chosen to grow in this weird way.
It's not contagious, it doesn't spread: some plants seem to be more susceptible to it than others, and I would certainly say that, anecdotally, Forsythia is the most likely suspect.
So what do you do if you find an example? Well, you don't have to worry about it: it's not infectious, it's not nasty in any way - if you don't like the look of it, prune it out, but I think it's actually rather an interesting little quirk, don't you?
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