Development of water-conducting cells in the antipodal liverwort Symphyogyna brasiliensis (Metzgeriales)

The thallus of the metzgerialean liverwort Symphyogyna brasiliensis Nets contains a strand of dead thick-walled tells with helicoidally-arranged pits that arc presumably involved in water transport. During the first phase of differentiation these cells undergo a 13-16-fold elongation while remaining thin-walled and almost unchanged in diameter. During subsequent maturation the walls become strongly thickened by deposition of highly electron-opaque material on extraplasmodesmal areas and of transparent material forming collars around plasmodesmata. Whilst the growing wall shows an ordered microribrillar texture and is strongly reactive to PATAg staining for carbohydrates, the material associated with plasmodesmata is amorphous and PATAg-negative. A dense cortical array of microtobules (MTs) overlies the growing wall except in proximity to plasmodesmata, which are closely associated with tubular endoplasmic reticulum (ER). During cellular maturation plasmodesmata undergo extensive secondary elongation by incorporation of cortical ER supposedly continuous with desmotubules. Quantitative analysis of plasmodesmal frequencies in relation to cellular elongation and wall thickness indicates that there is no de novo formation of plasmodesmata. Cortical MTs, wall microfibrils and secondarily-modified plasmodesmata are consistently co-aligned, all forming helices of about 45°. During maturation the Golgi apparatus proliferates and a vast number of vesicles containing PATAg-positive material are produced from a membrane domain interpreted as trans Golgi network, whilst PATAg-negative vesicles are formed along the fenestrated margins of C& and media) dictyosomal cisternae. Exocytosis of PATAg-positive vesicles is confined to extraplasmodesmal areas. In ageing cells abundant fibrillar material, also positive to PATAg-test, accumulates within pleomorphic membrane-bounded tubules. Final cytoplasmic dissolution involves the lysis of all cellular membranes and the liberation of the membrane-bounded fibrillar material, that is subsequently deposited onto the walls. The eventual dissolution of the plugs of amorphous electron-transport material results in the formation of open pits. Similarities in the cytological mechanisms underlying pore development in water-conducting cells of Symphyogyna and in the sieve elements of angiosperms are discussed.