Transparency shadow
-------------------
While the [Blender Manual page][1] says:

> For direct light sampling, use transparency of surfaces in between to produce shadows affected by transparency of those surfaces.

The tooltip is more clear:

[![enter image description here][2]][2]

With that option disabled, as in your case, the alpha channel of the texure won't be considered for shadows..

[![enter image description here][3]][3]


  [1]: https://www.blender.org/manual/render/cycles/settings/integrator.html#transparency
  [2]: https://i.sstatic.net/VuGYH.png
  [3]: https://i.sstatic.net/JS4HH.png