Let $L(n,d)$ denote the minimum possible number of leaves in a tree of order $n$ and diameter $d.$ In 1975 Lesniak gave the lower bound $B(n,d)=lceil 2(n-1)/drceil$ for $L(n,d).$ When $d$ is even, $B(n,d)=L(n,d).$ But when $d$ is odd, $B(n,d)$ is smaller than $L(n,d)$ in general. For example, $B(21,3)=14$ while $L(21,3)=19.$ We prove that for $dge 2,$ $ L(n,d)=leftlceil frac{2(n-1)}{d}rightrceil$ if $d$ is even and $L(n,d)=leftlceil frac{2(n-2)}{d-1}rightrceil$ if $d$ is odd. The converse problem is also considered. Let $D(n,f)$ be the minimum possible diameter of a tree of order $n$ with exactly $f$ leaves. We prove that $D(n,f)=2$ if $n=f+1,$ $D(n,f)=2k+1$ if $n=kf+2,$ and $D(n,f)=2k+2$ if $kf+3le nle (k+1)f+1.$
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