Other than general statements about ‘fruiting’ seasons, published floras provide little or no instructive information on moss phenology. Moreover, detailed primary data on reproductive cycles are limited to a very few mosses and remain unknown for the majority of the commonest species. Thus we recorded, over a three year period, the reproductive stages of five very common mosses (Bryum capillare, B. radiculosum, Grimmia pulvinata, Schistidium crassipilum and Tortula muralis) growing on walls in London, England, relying throughout on freshly observed materials rather than dried specimens used in most previous studies. In addition to all the stages visible to the naked eye, which we photographed at regular intervals, specimens were examined microscopically for the presence of viable gametangia, young embryos and the condition of the stomata.
Each species had its own distinct phenology and an unique sequence of capsule colour changes. In the two Bryum species, gametangium ontogeny, followed by fertilization, takes place in the spring but the embryos remain dormant until the autumn whereas these stages are autumnal in Grimmia pulvinata, Schistidium crassipilum and Tortula muralis with sporophyte development following immediately. Most stages in sporophyte ontogeny occur over the winter months. The time from embryo formation to spore release ranges from over fifteen months in the two Bryum species down to eight months in Schistidium. In all but this last species there is a delay of up to several months between sporophyte maturation and spore release. In Bryum, hygroscopic movements of the annular cells following heavy rain eventually leads to lid shedding. Over the three years of this study the reproductive cycles were generally the same except that damp weather in the autumn promoted capsule expansion in Grimmia and Schistidium and warm dry weather in the spring hastened capsule maturation in Bryum. Whatever the weather conditions, the stomata of the two Bryum species, Grimmia and Tortula were always open suggesting a primary role in capsule desiccation leading to spore discharge rather than the regulation of gaseous exchange.