I would think philodendron could be started by sticking a stem in water. Seems to me I've done that years ago. Or using the stem cutting method. I didn't think philodendrons were very hard to root.

Did you stick a stem with a "node", a joint looking feature of the stem, into the dirt? A leaf would be almost impossible to root outside a university laboratory unless you have a bit of stem with a node.

Several images at the link below. The positions may change but those about four through nine look pretty good at this time.

Roots in water never seem to me to be as healthy as roots in some solid. When I don't know for sure what I am expecting I get a clear plastic cup, add potting soil and water, and slip the cutting down the side so I can see when root formation begins. I sometimes set the cup within another solid cup so the root formation area will be dark.

Here is a link that might be useful: node images