Zur Herkunft und Struktur der Plasmafilamente in Assimilatleitbahnen

Publisher Website
Google Scholar
Abstract
Für die den Stoffleitbahnen eigenen fädigen Strukturen wird die Bezeichnung “Plasmafilamente” vorgeschlagen. Wie erste Untersuchungen zur Differenzierung der Siebelemente zeigen, entstehen die Plasmafilamente bei Dioscorea direkt im Cytoplasma. Ihre Verbreitungsweise und ihr Feinbau in frühesten Differenzierungsstadien lassen vermuten, daß sie unmittelbar aus dem Grundplasma hervorgehen. Das Netzwerk der Plasmafilamente ist stets durchsetzt von Elementen des ER und zu keiner Zeit von einer Membran umgeben. Das Einzelfilament hat bei Dioscorea, Primula, Cuscuta und Cucumis eine unbestimmte Länge und einen Durchmesser von 120–150 Å. Im Querschnitt zeigt es den Aufbau aus einem osmiophilen Ring mit einem elektronenlichten Binnenraum. Dem entsprechen in der Längsaufsicht zwei äußere kontrastreiche und eine innere kontrastarme Schicht. Der Aussagewert der Filamentfeinstruktur wird unter Berücksichtigung ähnlicher Strukturen und ihrer Bedeutungen in der tierischen und pflanzlichen Zelle diskutiert. Taking into account the literature on the so-called sieve-tube slime (“mictoplasm”, “slime strands”) and regarding its fine structure more in detail the term plasmatic filament (“Plasmafilament”) is proposed and will be used in this paper to characterize the individual exceedingly fine subunit of the plasmatic network (or slime) in sieve elements. Up to now plasmatic filaments have mostly been erroneously called “fibrils”. The dimension of a fibrill has now been defined anew and differentiated from its subunit “plasmatic filament”. In the first part of these investigations some aspects of the development of plasmatic filaments and their spreading over the total lumen of Dioscorea sieve elements will be reported. Previous to the first appearance of filaments the later sieve element abounds in plasmatic components, the groundplasm being extremely rich in ribosomes (Fig. 1). The difference between young sieve elements and the neighbouring parenchyma cells is nearly imperceptible apart from a slight variation in ribosome density. Plastids are very useful in distinguishing these two cell types from each other. The development of osmiophilic inclusions that characterize sieve-element plastids in Dioscorea has already been initiated in these very young cells. The earliest stages in the formation of plasmatic filaments that up to now have been revealed in Dioscorea show masses of filaments, some short and granular in appearance (Fig. 2: *), some already elngated and filamentous (Fig. 2: F). After expanding over the entire cell those filaments still look like having their origin directly in groundplasm (Fig. 5). Elements of the ER-system and many ribosomes cross the plasmatic filaments during all developmental stages of their network, which is at no time surrounded by any membrane. In sieve elements of Dioscorea, Primula, Cuscuta and Cucumis our investigations furthermore yielded some detail on the filament substructure. A cross-sectioned plasmatic filament is composed of an osmiophilic outer ring with a light centre (Fig. 11) corresponding in a longitudinal view to two deeply contrasted outer layers and an inner one without any contrast (Fig. 8). An individual filament has an overall diameter of 120–150 Å and an up to now indeterminable length that exceeds at least several microns. The real nature of these fine structures will be discussed in relation to similar structures and their meaning in plant and animal cells.