The question asks for the layer internal to the epidermis. In many botanical contexts, the cortex is the tissue layer located directly beneath the epidermis. However, the provided answer 'Epidermis' is logically circular. In Selaginella stems, the cortex is indeed the primary tissue found internal to the epidermal layer. This suggests a potential error in the provided answer key.

During meiosis, a diploid spore mother cell undergoes two successive nuclear divisions, resulting in the formation of four haploid daughter cells, which are the spores. This process is fundamental to the alternation of generations in plants.

The genus Selaginella is the primary taxonomic unit within the family Selaginellaceae. While the question asks for the genus, the provided answer 'Sellaginella' appears to be a misspelling of the genus name. In standard biological classification, the genus is the rank above species and below family. Selaginella is the sole living genus in its family.

While the stem of Selaginella possesses an epidermis and a cuticle, it typically lacks stomata. Gas exchange in the stem occurs through other mechanisms, as the absence of stomata is a characteristic feature of the stem anatomy in many species of this genus.

In Marsilea, a heterosporous fern, the spore mother cells within the sporangia undergo meiosis to produce a tetrad of four haploid spores. While many spores may degenerate in the megasporangium, the initial meiotic division consistently results in four spores.

The tapetum is the innermost layer of the sporangial wall. Its primary function is to provide essential nutrients and enzymes to the developing spore mother cells and spores. It eventually degenerates as the spores mature, releasing its contents to support their development.

Like most pteridophytes, Selaginella requires the presence of external water or moisture for fertilization. The motile, biflagellate antherozoids must swim through a film of water to reach the archegonium, where the egg cell is located, making moisture a critical limiting factor for their reproductive success.

Selaginella is a vascular plant that performs photosynthesis. Like most terrestrial plants, its vegetative parts, including the leaves covering the main stem and branches, contain chlorophyll, which gives them a green appearance. These leaves are typically small, simple, and arranged in a specific pattern along the stem to maximize light absorption for the plant's metabolic processes.

In the development of sporangia, certain archesporial cells differentiate into sporocytes, while others disintegrate. This disintegration process results in the formation of a nutrient-rich protoplasmic mass, which eventually contributes to the tapetal fluid. This fluid provides essential nourishment to the developing spore mother cells, ensuring their successful maturation into functional spores within the sporangial chamber.

Selaginella is classified under the class Lycopsida. However, the option 'Ligulopsida' is often used in specific older or alternative classification systems to group ligulate lycopsids. Note: There is a potential conflict as Lycopsida is the standard class, but the provided answer key selects Ligulopsida.