Final answer:
The structures on the underside of fern leaves that contain sporangia are called sori. They are distinct from the underground stems, which are rhizomes, and are essential for the fern's reproductive cycle, leading to the formation of gametophytes through released spores.
Step-by-step explanation:
The structures found on the underside of fern leaves that contain sporangia are called sori. These sori appear as small bumps on a fern's frond, which is a part of the plant's sporophyte stage. This stage is dominant in the fern's life cycle, featuring large compound leaves, or fronds, that serve both for photosynthesis and reproduction. The fronds may arise from underground stems known as rhizomes, which are different from sori and do not contain sporangia.
Inside the sori, you will find sporangia—structures that produce and release spores. When the spores are released into the environment, they may grow into haploid gametophytes if they land on a suitable substrate. The gametophytes produce gametes that can unite and eventually give rise to a new sporophyte, continuing the fern's life cycle.