In this videotutorial Greg Zaal explains how to use alpha maps in Cycles.
Greg Zaal writes:
In this tutorial you’ll learn how to use an alpha map in cycles to make something transparent in certain parts.
We’ll be making a leaf by creating an alpha map from the original leaf texture to use as a mask between a diffuse shader and a transparent shader.
This tutorial assumes you have some basic knowledge of how to use Blender and cycles.