  A vertex of degree one is called an end-vertex, and an end-vertex of a tree
is called a leaf. A tree with at most $k$ leaves is called a $k$-ended tree.
For a positive integer $k$, let $t_k$ be the order of a largest $k$-ended tree.
Let $\sigma_m$ be the minimum degree sum of an independent set of $m$ vertices.
The main result (Theorem 2) provides a lower bound for $t_{k+1}$ in terms of
$\sigma_m$ and relative orders: if $G$ is a connected graph and $k$, $\lambda$,
$m$ are positive integers with $2\le m\le\min\{k,\lambda\}+1$ then either
$t_{k+1} \ge \sigma_m +\lambda(k-m+1)+1$ or $t_k\ge t_{k+1}-\lambda+1$.
