Selaginella

Selaginella (Division Lycophyta, Order Selaginellales), commonly called 'spike moss', grows in humid temperate and tropical rainforests; S. rupestris and S. lepidophylla are xerophytic, and a few species — famously S. lepidophylla — curl up and appear dead in drought, then uncurl and green up again with moisture, earning the nickname 'resurrection plant'.

The sporophyte (2n) is differentiated into root, stem and leaves, with habits ranging from prostrate/creeping (S. kraussiana) to suberect (S. rupestris), erect (S. erythropus), climbing (S. alligans) or even epiphytic (S. oregana). Based on stem structure and leaf arrangement, the genus splits into two subgenera: Homoeophyllum (erect stem, spirally arranged, uniform leaves — S. rupestris, S. oregana) and Heterophyllum (prostrate stem with short erect branches and two distinct leaf sizes — dorsal microphylls and ventral megaphylls — S. kraussiana, S. lepidophylla). Adult roots are adventitious, arising either at points of dichotomy or from a leafless, unbranched, downward-growing organ unique to the genus, the rhizophore, which produces tufts of adventitious roots at its tip. Leaves are microphyllous, sessile, single-veined, and each carries a small membranous tongue-like structure on its upper surface called a ligule (with a basal cushion of cells, the glossopodium) whose exact function is debated but is thought to help with water absorption/secretion and preventing desiccation.

Internally, the root shows an epidermis, a homogeneous parenchymatous cortex, an endodermis and a monarch, exarch protostele; the rhizophore is broadly similar but its cortex is layered (outer sclerenchyma, inner parenchyma). The stem shows a striking feature: its protostele is suspended in an air space (created by the cortex stretching faster than the stele during growth) and held in place by radially elongated endodermal cells called trabeculae; the number of steles varies from monostelic to distelic or even polystelic depending on species, and while tracheids are the usual conducting cells, true vessels occur in S. densa and S. rupestris. The leaf shows upper and lower epidermis with stomata on both surfaces, loosely packed mesophyll, and a single median vascular bundle. …