What are the characteristics of triode?

With a circuit diagram, explain how the static characteristic curves of a triode are drawn?

Show how the tube constants are estimated with the help of these curves.

Why the vacuum diode does not work under reverse bias condition?

The $\mathrm{V}-\mathrm{I}$ characteristic of vacuum diode as shown below:

You have to identify the regions which marked as $\text{'}\mathrm{a}\text{'}$ and $\text{'}\mathrm{b}\text{'}$.