You have to use an alpha to cut the white part out. It is a fourth channel in the colors and when you create it by default it will be an alpha channel. Make sure that the part you want to show is white and the part on the alpha you want to cut away is straight black. Anything other than straight black and it will have a silohette. Make sure you save it as a tiff or a tga for those will keap an alpha channel while a jpg will not. If you use mental ray make sure you use a tga because it has problems with tif files.


With leaves, if you want to get tricky, you could have a translucence channel that allows light to go through the leaf part but not the ribs and veines.

Hope this helps.