The basal portion of a mature ligule of Selaginella is called as: 

The correct answer is: Glossopodium.

Explanation:

• Glossopodium is the basal portion of the mature ligule in Selaginella, which is a genus of clubmosses. The ligule is a small outgrowth at the junction of the leaf and the stem in some vascular plants, including Selaginella. The Glossopodium is particularly important for distinguishing the type of ligule in Selaginella from that of other plants.
• Basal sheath: This term usually refers to a structure that surrounds or is at the base of a plant part but isn't specifically related to the ligule in Selaginella.
• Lamella: This is typically a thin plate-like structure found in various biological contexts, including certain types of plant tissues, but not specifically the basal portion of a ligule.
• Stalk: In botanical terms, a stalk generally refers to a supporting structure like a stem or pedicel but is not used to describe the basal part of the ligule in Selaginella.
• The ligule in Selaginella plays a role in protecting the growing tips of the plant, and the Glossopodium is a distinctive feature of its ligule, helping in the identification of species within the Selaginella genus.