Binary Tree - Random Generator

Suppose I have a Binary tree like this -

       5               
      / \       
     /   \      
    /     \     
   /       \    
   2       8       
  / \     / \   
 /   \   /   \  
 1   3   6   9   
      \   \   \ 
      4   11   10 

Now I have a random generator which will generate a number between 1 to size of tree (10 in this case). Based on the random value generated by the random generator I have to return the node from the tree(Suppose, generator given 7, so I return the 7th node(value 11), doing inorder traversal). Tomorrow I add 4 more nodes to the tree. How do I maintain the consistency? As in, the same node from the tree is returned for the random value. The inorder traversal will create a different array and the value of the index will change.