How do I use namespaces with TypeScript external modules?

I have some code:

baseTypes.ts

export namespace Living.Things {
  export class Animal {
    move() { /* ... */ }
  }
  export class Plant {
    photosynthesize() { /* ... */ }
  }
}

dog.ts

import b = require('./baseTypes');

export namespace Living.Things {
  // Error, can't find name 'Animal', ??
  export class Dog extends Animal {
    woof() { }
  }
}

tree.ts

// Error, can't use the same name twice, ??
import b = require('./baseTypes');
import b = require('./dogs');

namespace Living.Things {
  // Why do I have to write b.Living.Things.Plant instead of b.Plant??
  class Tree extends b.Living.Things.Plant {

  }
}

This is all very confusing. I want to have a bunch of external modules all contribute types to the same namespace, Living.Things. It seems that this doesn’t work at all — I can’t see Animal in dogs.ts. I have to write the full namespace name b.Living.Things.Plant in tree.ts. It doesn’t work to combine multiple objects in the same namespace across file. How do I do this?