On Material Tab, go to Transparency Section, activate leaves' Material transparency, set it's transparency mode to "Z Transparency" and then set the Alpha value to 0.
On Texture Tab, go to Influence Section, tick Alpha and set it's value to 1. Don't forget to tick Use Alpha on Image Section.
If you use an opaque image with transparency mask image, you can make the transparency mask to overlay the opaque image one using Texture Index and do things above (But, I don't guarantee it will always work).
if material still black in render mode, On Material Tab go to Shadow section tick Receive Trasparent e don't tick Cast.