How can I get a List from all nodes in a tree using LINQ?

My classes are: 

```
class Node
{
 public class Node()
 {
  Children = new List<Node>();
 }

 public List<Node> Children { get; set;}
}

class Tree
{
 public Tree()
 {
  Roots = new List<Node>();
 }

 List<Node> Roots { get; set;}
}

```