I have a TreeView on a Grid and I am trying to make the Items of the tree stretched all over the grid's width.

<TreeView x:Name="tree">
<Button>Button 1</Button>
<Button>Button 2</Button>
<Button>Button 3</Button>
<Button>Button 4</Button>

I just can't make that happen unless I set each item's size myself.
How can I make the panel's layout system work on the tree's items?