Final answer:
The term for the outer layer of a lichen is the cortex, which protects the lichen and is one of several layers in its structure. Lichens can have varied appearances influenced by their photobionts, and the cortex is an essential protective feature.
Step-by-step explanation:
The term for the outer layer of a lichen is cortex. A lichen thallus typically consists of several layers, including the upper cortex made up of tightly packed fungal hyphae, which serves as protection. Below this layer, there is a layer containing the photobiont like algae, where photosynthesis occurs, followed by the medulla. The lower cortex, similar to the upper one in composition, also provides protection, and at the very bottom, rhizines are found, which function to anchor the thallus to its substrate.Lichens may exhibit various forms, such as crust-like, hair-like, or leaf-like structures, contributing to their adaptability in different environments. The appearance of lichens can be colorful, with the coloration usually determined by the type of photobiont present within the layers of the thallus.
The correct term for the outer layer of a lichen is the cortex. A lichen is a symbiotic organism made up of a fungus and an alga or cyanobacterium, and the cortex is the tightly packed layer of fungal hyphae at the outer surface of a lichen that provides protection. The cortex is located above the medulla, which is the inner, loosely packed layer of fungal hyphae.