Peristome Development in Mosses in Relation to Systematics and Evolution. IV. Haplolepideae: Ditrichaceae and Dicranaceae

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Abstract
The classification of mosses into subclasses and orders is based to a large extent on the morphology of the peristome teeth, yet little information exists on development of peristomes. In this paper we describe the sequences of cell divisions leading to peristome formation in six species of mosses representing two families with haplolepideous peristome structure. The earliest developmental stages conform to the pattern documented in other mosses and highlight the uniformity of these early stages among mosses of diverse relationship. Peristome development in haplolepideous mosses diverges from the pattern found in diplolepideous species when each of eight cells in the Inner Peristomial Layer undergoes anticlinal divisions. Haplolepideous species share a pattern of peristome development that appears to be unique to this group of mosses. Similarities in the arrangement of cells comprising mature haplolepideous peristomes and early stages of Bryum-type diplolepideous peristomes may indicate a relationship between these peristome types, or may be due in part to evolutionary convergence. Developmental evidence suggests that haplolepideous peristome teeth are homologous to the cilia of Bryum-type peristomes rather than to the endstome segments, as is commonly thought.