It's hard to pick just one and I do love the ones that have already been mentioned. Howard is always funny.
Was just watching "From Here to Paternity" the other day and liked the bit where Barry and Glenda are out running and sit down to rest by a wall (on the other side from Howard and Marina, of course!) As Barry and Glenda stand up, after hearing voices, Howard doesn't miss a beat but launches straight into "teacher of the rudiments of botany" mode.
"And there to the right, you can see the common lesser winthrop, which is easily recognised by its hairy petals. Now, if we move along, we should be able to find some examples of the Stringwort. Now, the stringwort is always easily recognisable by the arrangement of leaves on the stem..."