If I understand you, Id suggest creating a sphere (or a series of them at staggered positionings) and texture all of them with a leaf texture that has an alpha channel for the spaces -- in B4, therere two such textures in the miscellaneous section, I think. Shafts of light through the leaves? I believe its possible in B5, but I dont know of anything in B4 that will create such an effect without dragging your rendertime into the ground.