How can I make transparant objects with a png texture I'm new to Blender, and I have imported a file with a number of vertical planes on which I have put a transparant tree texture, see picture. The planes are intersecting and I have put a png with alpha channel on it However, whatever I try in the material editor, the I can't make the polygons transparanet so that only the tree image is shown. I must oversee something obvious, but does anybody have any idea on how to fix that?
(This answer is for Blender Render, not Cycles, as you seem to have that renderer enabled in your screenshot).

First, on the texture settings panel, look for "Influence" and put a checkmark on "Alpha":



Then, back on the material settings panel, put a checkmark on "Transparency".

Finally, and this is a key step, set "Alpha" to zero here on the material transparency panel.  Think of this as a kind of minimum alpha for the material: The provided texture can make it more opaque, not less.



The results should work in the Material and Rendered viewport shading modes, and if you've selected the same image as the active image in the UV editor, it can work in the "Textured" viewport mode as well.