Added: 2011-01-21An Elm, but with rather strange bark. Unfortunately this tree has now died (these photos were taken in 2003 and 2004). Near Cambridge, Ontario.
Various leaves.
The number of veins and the number of forked veins suggests White Elm (Ulmus americana), per the book Trees of Canada by Farrar.
Upper surface of a leaf.
Leaves on the tree.
The bark is rather strange. VERY prominently ridged. This led me to wonder if it was a Rock Elm (Ulmus thomasii). But the branches showed none of the characteristic ridging of that species.