Interpolation theorem for the number of end-vertices of spanning trees

The following interpolation theorem is proved: If a graph G contains spanning trees having exactly m and n end-vertices, with m < n, then for every integer k, m < k < n, G contains a spanning tree having exactly k end-vertices. This settles a problem posed by Chartrand at the Fourth International Conference on Graph Theory and Applications held in Kalamazoo, 1980.