Peristome reduction

Preperistome plates adhere to the outer face of the exostome and are formed by deposition in the cell layer external to the OPT. These plates were previously used to delimit Mesonodon (Buck, 1980 Gradstein et al., 2001). However, these structures have been found to occur sporadically in a range of species in both Entodon and Erythrodontium and to be lacking in many specimens of Mesonodon (Figure 12.2D). They appear to be an occasional by-product of peristome reduction and of no taxonomic significance in this family. 

Reduction or absence of endostomial cilia is commonly associated with peristome reduction. Residual cilia in Entodontaceae are only consistently found in E. stereophylloides where they have the same range of ornamentation as the endostome segments. Ciha remnants are only rarely found in the other species in Entodon subgenus Entodon. 

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 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. 

Kungu, E. M., Bonner, L. and Longton R. E. (2003) Patterns of peristome reduction and ornamentation in African Entodontaceae. Journal of the Hattori Botanical laboratory, 93 223-246. 

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. 

Character Reduction and Peristome Morphology in Entodontaceae Constraints on an Information Source... 

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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