In this article we will discuss about the morphology of selaginella with the help of diagrams.
Selaginella with about 700 species is worldwide (tropics to subarctic) in distribution. It is abundant in tropics, with a wide range in growth habits and habitats.
Many species are flat creepers (Fig. 8.1 A) on the ground (S. kraussiana). In some species the plants are small and upright, like moss and others produce moss-like upright branches (5. rupestris). Rarely, the plants are quite erect, several feet tall (S. viridangula), or vine attaining a length of up to 20 metres.
Selaginella spp. is cultivated in greenhouses because of the beauty of their branches and foliage and is commonly known as ‘small club-mosses’. In a greenhouse one can see an array of colours, dark green to light green and bluish green to iridescent.
The majority of the species are aniso- or heterophyllous and are placed in subgenus Heterophyllum. Only about 50 species are iso- or homophyllous forming subgenus Homeophyllum. Stems in heterophyllous forms are generally prostrate with short erect branches and the leaves are arranged in four rows; two rows of small leaves on the upper side and two lateral rows of larger ones (Fig. 8.1 A, B, C). In homeophyllous forms the plants are generally erect and the leaves are spirally arranged.
Tropical species inhabit moist rainforests and constitute the dominant element of forest floor. Some forms are xerophytes; grow on exposed rocks, with ability to survive desiccation and are referred to as ‘resurrection plant’ such as S. lepidophylla (Fig. 8. 1D, E). This species has adapted to grow in Mexican desert. During dry season the plant curls up in the form of a ball and rejuvenates with rain.
The leaves are sessile with a single unbranched vein, traceable to stem stele. In or near the axil of each leaf (vegetative or sporophyll) there is a laminate outgrowth, the ligule, which differentiates and matures very early in the ontogeny of leaf. A mature ligule is tongue- shaped to fan-shaped (Fig. 8.2A).
Its basal region is made of tubular hyaline cells, forming the sheath. Following the sheath is the hemispherical region of thin and greatly vacuolate cells referred to as glossopodium (Gr. glossa = tongue, podium = foot). The remaining cells are isodiametric, the apical region is one-cell thick and is made up of elongate cells with scanty contents.
The function of ligule is unknown. It develops very early and matures long before its associated leaf matures. It is thought to be concerned with conservation of water and prevents shoot desiccation. It is also suggested to be concerned with upward movement of inorganic salts by compensating for smaller and less effective leaf primordial.
This is supported by structural similarity of sheath cells and cells of endodermis. Also there is presence of transfusion tissue (tracheid-like cells) between sheath cells and the vein of leaf. There is evidence of secretion of mucilaginous substance by ligule.
Branching in Selaginella is characteristic, terminal and unequal, forming weaker and stronger shanks (Fig. 8.1A). At each dichotomy there are one or two meristems on either side. These angle-meristems develop into cylindrical outgrowths described as ‘rhizophores’.
In most of the species, only one of the angle-meristems, i.e., ventral one is active, while the other remains as dormant papilla. In some species both the angle-meristems remain dormant especially on vertical axes. A new species of this plant S. moratti from central Madagascar is unique in having rhizophore tubers.