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

This tribute* to Alan Crundwell and Harold Whitehouse provides the first il lustrated study of the development of gemmae in Dicranoweisia cirrata and O rthodontium lineare, and the first account of the gemmiferous protonema. of O. gracile. Foliar and protonemal gemmae in D. cirrata are absent or spars e 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 pollute d areas. In contrast, O. lineare produces gemmiferous protonemata on all it s natural substrates and nutrient dilution has little effect on their devel opment in culture. D. antarctica and D. crispula, non-gemmiferous in nature , also failed to produce gemmae in culture. Unlike most moss gemmae, germin ation in D. cirrata is apolar. Orthodontium produces both terminal and inte rcalary gemmae, but only the former occur in Dicranoweisia. Detachment invo lves 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 protonemat a. produce a substance that elicits gemma formation and inhibits their germ ination in situ. Protonemata of O. gracile are more prone to redifferentiat e into thin-walled brood cells and are, therefore, probably more desiccatio n-sensitive than those of O. lineare. This may be a major reason why O. gra cile is being over-run by the introduced species.