Is the entire working copy “at one branch”?

This is a very basic question but maybe I just don't know how to
phrase my query well enough to find an answer.

Let me begin with a SVN example. SVN doesn't have a "current branch"
as a property of a working copy, so I'll give the example with
revisions instead. Consider a working copy with the dirs `foo` and
`bar` in it. In SVN one can `update` foo to an earlier version, but
keep `bar` at `HEAD`.

Is it possible in an analogical git working copy to checkout one
branch in dir `foo` without changing the branch checked out in `bar`?

If not, why not?

TIA,
Stefan Monov