Shading a patch that is behind another one in the light direction

I am trying to make a lighting model on a set of leaves, and I am modeling the leaves as patches in Matlab. Unfortunately, when I have to 'leaves' behind each other, they both get lit, even if technically the one behind should be shaded (see image at https://dl.dropbox.com/u/43209398/Summer_Proj/results/shading.jpg). I've been trying to google this, but I couldn't find anything useful.

I am setting the light like this,

    light('Position',lightPos,'Style','local');

The patches have, for now, these properties:

    'FaceColor','green', 'EdgeColor', 'none',...,
    'FaceLighting','phong',...
    'SpecularExponent',1,...
    'SpecularColorReflectance',0.2,'SpecularStrength',3,...
    'FaceAlpha', 1,'AmbientStrength',0.0,...
    'DiffuseStrength',0.0

Also, if I set 'FaceAlpha' to something smaller than 1 (eg 0.97), my images look completely different:

for 1: https://dl.dropbox.com/u/43209398/Summer_Proj/results/one.jpg

for .97: https://dl.dropbox.com/u/43209398/Summer_Proj/results/point97.jpg

The results for the higher transparency level are closer to what I want, but the leaves still don't shade each other.

Any help would be appreciated. Thank you!