Exostome ornamentation

Previous studies on variation in peristome morphology include an intergeneric snrvey of endostome variation in 70 species of diplolepidous mosses (Shaw and Rohrer, 1984), and stndies of Bryaceae (Shaw, 1985) and Brachytheciaceae (Ignatov et al., 1998). The infraspecific variation in the exostome ornamentation in the reduced peristome of Pterigynandrum flliforme had resulted in a number of varieties being described (Buck, 1980). 

Ornamentation of the dorsal plates in the basal third of the exostome teeth conspicuously striate (striae set with papillae or not) state 1 smooth, papillose, or weakly striate near the tooth base state 2. 

Taxonomic recognition does not just depend on broad character states of exostome papillose or exostome striate. Within these categories, there are often subtle differences in ornamentation... 

Vitt (1981) commented on the correlation between loss of function, and fusion or reduction in the peristome, concluding that loss of function precedes peristome reduction or fusion in xerophytic environments. However, observation of all species of African Entodontaceae proved that these peristomes retain some residual function. Even in those genera with extreme peristome reduction the remaining exostome remnants respond to changing levels of humidity, and when closed prevent spore release. Exostome function is related to both the internal anatomy and chemistry of the exostome and the form of the external ornamentation. 

The peristomes of Erythrodontium often have an exostome truncated at or just above the horizontal striations. This was particularly common in E. julaceum (Schwaegr.) Par Erythrodontium barteri (Mitt.) Broth, has a consistent pattern of ornamentation, with the horizontal and vertical ornamentation overlain with papillae, fusions and occlusions and forming a distinctive pattern on the OPL combined with a consistent reticulate ornamentation on the upper exostome PPL (Figure 12.2C and Figure 12.3B). 

Interpretation of the range of ornamental patterns found within the family as currently recognized is also problematic. Are these ancestral states that predate the evolution of reduction Given the complexity of the internal anatomy of the striate and papillose exostomes, it is tempting to consider these as ancestral. The alternative hypothesis that the peristome evolved the reduced state and concurrently or subsequently evolved these different ornamental patterns together with the marked internal differentiation seems contradictory. 

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