What is the hollow stem of a grass plant called?

The hollow stem of a blade of grass is called a culm. They are rarely branched above the ground. The second half of the leaf then opens out into the leaf blade. But there is more to this in plant physiology. There are four main parts to a grass plant, the roots, the stem or culm, the leaves and flowerhead which is properly called, inflorescence.

You can go further and examine each part for its diagnostic features. These help in correct identification. In complex cases, the combination of several characters can lead to the correct name.