The underground stem of a fern is called a rhizome. The rhizome is a short, vertical stem located near the soil surface. It is hard to see due to its length, unlike other plants that have visible, aerial stems.
Because the rhizome is very short, the leaves are the more visible parts of a fern. These leaves are called fronds and are often clumped together. They die at the end of each growing season. Fronds differ greatly in size, growing up to 3 feet in length in some species. Fronds may also grow along the rhizome, which is less common than the clumped formation.