Is there a way to change a varable in another plugin from with in a plugin?

Example Frist plugin has  public string trychange = "Changethis";
how can i change this in another plugin with out adding this to the first plugin?

void testthis(string newtxt)
{
    trychange  = newtxt;
}

Is there a way to avoid the adding of a call like that?

Yes, but only by calling a method that sets it. You can't set a variable directly.