Studies of protonemal morphogenesis in mosses. VIII. The gemmiferous protonemata of Orthodontium and Dicranoweisia

Abstract

This tribute to Alan Crundwell and Harold Whitehouse provides the first illustrated study of the development of gemmae in Dicranoweisia cirrata and Orthodontium lineare, and the first account of the gemmiferous protonema of O. gracile. Foliar and protonemal gemmae in D. cirrata are absent or sparse on nutrient-poor substrates and most abundant only on nutrient-rich barks. Development of gemmiferous protonemata in culture only on nutrient-rich medium underlines the field observations that gemma production is nutrient dependent. This may well be a reason why this species flourishes in polluted areas. In contrast, O. lineare produces gemmiferous protonemata on all its natural substrates and nutrient dilution has little effect on their development in culture. D. antarctica and D. crispula, non-gemmiferous in nature, also failed to produce gemmae in culture. Unlike most moss gemmae, germination in D. cirrata is apolar. Orthodontium produces both terminal and intercalary gemmae, but only the former occur in Dicranoweisia. Detachment involves the formation of slightly elongate aplastidic tmema (abscission) cells in Dicranoweisia but either very short tmema cells or separation along the middle lamella akin to brood-cell liberation, in Orthodontium. Experiments with activated charcoal and old media reveal that Dicranoweisia protonemata produce a substance that elicits gemma formation and inhibits their germination in situ. Protonemata of O. gracile are more prone to redifferentiate into thin-walled brood cells and are, therefore, probably more desiccation-sensitive than those of O. lineare. This may be a major reason why O. gracile is being over-run by the introduced species.