Peristome reduced

Howard Crum (Figure 1.10) and I (Buck and Crum, 1990) relied totally on gametophytic characters in defining familial boundaries of the Leskeaceae/Thuidiaceae complex even though traditionally the families were separated on sporophytic differences (sometimes even into different orders). We allowed, within a single family, sporophytic evolution in which fnlly developed hypnoid peristomes can become greatly reduced. We correlated these reduction seqnences with differences in habitat. 

Species of Heterocladium differ markedly from the Neckeraceae in having perfect double-alternate peristomes (versus moderately to strongly reduced), julaceous to slightly complanate... 

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). 

The exostome is formed from the common periclinal cell wall pair between the inner wall of the OPL and the outer wall of the PPL. The endostome is formed from the common periclinal cell wall pair between the inner wall of the PPL and the outer wall of the IPL (Figure 12.1). Autolysis of the anticlinal walls of the cells between the exostome and the endostome separates the two rings of the diplolepidous peristome, while autolysis of the periclinal walls between adjacent teeth separates the structures within each ring of the peristome. In reduced peristomes this autolysis may be incomplete, leaving adjacent structures partially attached. 

The evolution of reduction is acknowledged as an advanced state recurrent across a wide range of acrocarpous and pleurocarpous taxa. In particular peristome reduction is consistently associated with the adoption of the epiphytic habit. This convergent evolution across snch a broad taxonomic spectrum indicates a strong adaptive advantage. The value of this suite of reduced aud co-occurring character states in the peristome as synapomorphies defining the family is therefore called into question. 

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. 

There must be some reservations about the monophyly of a family united on the basis of cooccurring characters of erect capsule and reduced peristome, since this combination occurs repeat-... 

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