Leaf: Simple, alternate on stem; small, broadly oval, short-pointed at end and finely toothed along margin; green and shiny above, dull green below; usually ranges in size from 1" to 2" but often 4" or more in length as well as in width on vigorous young shoots. Leafstalks are flattened at right angles to leaves, causing leaves to quake or tremble in a very slight breeze.
For more information see
http://www.dnr.state.mn.us/trees_shrubs/deciduous/quakingaspen.html.