Ignatov M.S., Spirina U.N., Kolesnikova M.A., Volosnova L.F., Polevova S.V., Ignatova E.A. 2018. Buxbaumia: a moss peristome without a peristomial formula
Arctoa (2018) 27: 172–202
This study of peristome development in Buxbaumia aphylla, B. minakatae and B. viridis has revealed major differences from all other peristomate mosses. The fundamental square patterning, characteristic of moss peristome formation, is lacking in Buxbaumia. Endothecium and amphithecium are differentiated, but cells of the endothecium are arranged radially and are likely derivatives of a large tetrahedral central cell. Cells of the inner peristomial layer (IPL) are all offset against cells of the primary peristomial layer (PPL), in both transverse and longitudinal sections. At early stages, IPL cells are 1.2–2 times fewer than PPL cells; subsequently they become about equal in number. However, close to urn base additional anticlinal divisions result in IPL cells double in number to those in the adjacent PPL layer. The maximum number of cells in the PPL is 48 in B. aphylla, but the peristomial formula can be calculated only by counting of all cells in moderately regular cell rings around endothecium and dividing these by eight. However the absence in Buxbaumia of cell columns, as in other mosses, and the progressive increase in the cells in peristomial layes makes such a count too approximate and uncertain. The pleated cone in Buxbaumia minakatae and in the distal half of B. aphylla has about 24 folds, whereas closer to base the folds are more numerous, with up to 40 folds. The cone-shaped endostome points the close relationship to Diphyscium, and also their prostome structure is very similar. However Diphyscium sporophyte at early stages of development is narrow and its peristomial layers differentiate as in other mosses and have similar to haplolepideous mosses cell arrangement. Contrary, in Buxbaumia the urn is broader than long at this stage and IPL forms continuous subepidermal layer overarching semi-globose endothecium. Its development proceeds essentially by means of morphologically longitudinal / spatually almost radial cell divisions, but not periclinal ones as in Diphyscium and all other mosses. This mode of development explains the progressively increasing number of cell in IPL, and consequently also PPL and OPL. The sporophyte structure in Buxbaumia does not represent an archaic type, but seems to be extremely specialized.