Hair and Particle Systems - Blender Master Class: A Hands-on Guide to Modeling, Sculpting, Materials, and Rendering - page 133

Figure 9-14: Using a texture to control hair particles. Here, a procedural Voronoi texture has been used to affect particle
length. You can use both procedural and image textures to affect particle properties.
Figure 9-15: Simple grass models.
These are all just planes and cylinders
that are subdivided, stretched, and
grabbed with proportional editing to
form them into the stems and leaves
of the grass. Each is a separate object
with its origin at the base of the plant.