If you saw this sign, what kind of plant(s) would you expect to be crossing?
In this context, plant refers to “a large, heavy machine or vehicle used in industry, for building roads, etc.” It can also mean “machines used in industry” or “a factory in which a particular product is made or power is produced” [source]
Apparently the first recorded use of plant for a factory dates from 1789 – this meaning developed from the idea of the factory being ‘planted’ [source]. Perhaps the meaning was extended to the machines used in factories, and to other large industrial machines.
Is plant used to refer to large machines only in the UK?
In US, “plant” is commonly used to denote “a place of production of something”. However, I have never heard it used for machinery alone and have never seen anything like that sign. I should put it by my desk since I have a lot of plants growing.
I’ve never seen plant used that way in the US. For us, it is used for the factory’s campus rather than for the machinery.
My mom was recently in Scotland and sent me pictures of anything she found unusual, which included lots of signery. There were some signs like “Haste ye aback” and “Dunnae chuck bruck” that I’ve seen, which are definitely Scots, but I was puzzled by the “heavy plant crossing” and suggested that Ents must be native to the UK, after all, what other kinds of heavy plants cross? Good to know the proper meaning of the sign.
Perhaps it is referring to overweight Welsh children…
Plant in Welsh is a collective noun meaning ‘children’ (unit form: plentyn). It is cognate with Irish clann (‘family’) but apparently also with Latin planta (‘plant’). We speak of ‘family trees’, so perhaps the connection is not all that obscure.
The sense of ‘industrial machinery’ is a bit of a mystery, but I imagine it is derived from the sense of a ‘factory’ – a place of conversion and production, much as a plant takes water and nutrients from the soil and converts them into leaves, branches, fruit etc.