Splachnaceae is a family of mosses, containing around 70 species in 6 genera. Around half of those species are entomophilous, using insects to disperse their spores, a characteristic found in no other seedless land plants.
Many species in this family are coprophilous, growing exclusively on animal faeces or carrion. For this reason, certain genera such as Splachnum Hedw. are often referred to as dung mosses.
Description
Gametophyte
Mosses in this family are predominantly dioicous (archegonia and antheridia on separate individuals); although exceptions are known such as S. pensylvanicum which is monoicous. The gametophyte is always [[acrocarpous (standing up), with green-yellow to reddish leaves/stems, and most often under 5 cm in height. Stems stand vertically and when cross-sectioned, can be seen to have a well-defined central strand. Large parenchymatous cells surround the central strand with thin, red to orange cell walls. Cortical cells are often more red in color, being narrow and thick-walled. Axillary hairs may be present along the stem, although axillary branches are infrequent. Rhizoids are dark brown or red and may be papillose.
Female plants are more robust than male plants; having spreading, ovate to lanceolate leaves that are moderately crisped when dry. Leaf distribution may be homogenous or may become larger/denser near the apex of the stem. Leaves may be serrate (finely toothed) on the edges, and possess a single costa that often ends before the apex. Laminal cells are thin-walled, being more rectangular towards the base and hexagonal at the apex. Perichaetial leaves are often larger, but similar in structure to the other leaves on the stem.
Male plants are more slender, with a looser leaf distribution. Leaves differ in size and differentiation towards the apex of the stem forming an often-bulbous perigonia. Perigonial leaves are strongly differentiated, being ovate and tapering to a long tip. The perigonium is terminal, and will often harbour paraphyses.
Early stages
Like other mosses, gametophytes start as a haploid spore which quickly germinates to become a uniseriate protonema. This protonema serves members of Splachnaceae by quickly colonizing its preferred substrate, developing in three stages: the chloronema, caulonema, and finally the leafy gametophore.
Spores
Unicellular spores are produced through meiosis by the sporophyte. In Splachnaceae, they are often small and sticky for easy insect dispersal. Spores are sometimes dispersed in clusters.
Chloronema
The chloronema is the earliest stage of protonema, having unique features such as irregular branching, round chloroplasts, and transverse crosswalls. There is no budding at this stage.
Caulonema
The caulonema is the secondary stage of protonema. It is branched regularly, has spindle-shaped chloroplasts, and oblique crosswalls. At this stage, budding begins.
Sporophyte
Many species in this family have very exaggerated sporophytes that are highly adapted for their specific ecological relationships.
The seta of Splachnaceae is usually elongate and erect, with a defined central strand. The sporangium is highly variable in shape. In many species, the middle of the sporophyte (hypophysis/apophysis) may be highly inflated or flared in order to attract insects. Above this is a shorter "urn" which is the same colour, and harbours a bluntly conical or convex operculum on top; of which the annulus is poorly developed. A single peristome is present, with 16 variably pigmented teeth (exostome usually consists of 8 teeth). Stomata are often abundant on the sporangium.
A haploid calyptra, composed of tissue from the gametophyte, may be present on the sporophyte; it being nearly always mitrate (shaped like a bishop's cap) and smooth.
Taxonomy
Splachnaceae currently resides in the order Splachnales (M. Fleisch.) Ochyra; which is further nestled within the class Bryopsida. It comprises 6 genera and approximately 73 species. The type genus is Splachnum Hedw.
Three subfamilies have been named within the Splachnaceae.
Splachnoideae (Aplodon, Splachnum, Tetraplodon) can be distinguished by their highly differentiated and often inflated hypophysis. All species are coprophilous and entomophilous excluding T. paradoxus, which is only coprophilous. Genera are distinguished by their peristome teeth.
Voitioideae (Voitia) is characterized by the absence of a differentiated line of dehiscence on the sporangium (cleistocarpy). Spore dispersal is only achieved following the decay of the sporangial wall. Species in this subfamily are coprophilous but not entomophilous.