I really thin kit depends on A) if the plant has a clear owner and B) how accesable is the particular plant to others.
If the plant is something that someone is raising for profit, eg a tree at a tree farm, fruit or vegetables on a farm, etc, that is certainly wrong.
If the plant is in a public area where the public would suffer (and I know I’m using the word very lightly) if everyone else did as you did, that wouldn’t be right. For example, if a neighborhood has 6000 people and only one tree, don’t take a leaf, because if everyone did there would be no tree left. This assumes you don’t have permission from the tree owner (most likely the city). If you do, go ahead and do as you will.
If you’re in a remote public area, let’s say you’re hiking in the desert on public property, far off the beaten path, where uyou are the only person who will ever see the plat, I see no reason not to do as you will, unless the plant is protected for some reason.