I think that with all the unicodes lying around these days we can do a little nicer:

                ┌───┐
          ┌─────┤ 1 ├─────┐
          │     └───┘     │
        ┌─┴─┐           ┌─┴─┐
      ┌─┤ 2 ├─┐       ┌─┤ 3 ├─┐
      │ └───┘ │       │ └───┘ │
    ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐
    │ 4 │   │ 5 │   │ 6 │   │ 7 │
    └───┘   └───┘   └───┘   └───┘

(Better if the presentation doesn't lead beyond the font.)