Plasmalemma invaginations as characteristic constituents of plasmodia of Physarum Polycephalum

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Abstract
Plasmodia of Physarum polycephalum giown on agar or filter paper and fed with rolled oats as food or with a partially defined medium were morphologically analysed in the living state and after fixation. Observation of the living plasmodium growing on agar reveals plasmalemma indentations in the outer regions of protoplasmic strands, which were studied in more detail by phase-contrast microscopy of unstained 1-μm sections. Plasmodia fixed and embedded in situ, i.e. in close contact to their substrate, exhibit an extensive system of plasmalemma invaginations as characteristic constituents throughout all regions. In plasmodial strands measuring between 40 μm and 1·5 mm in diameter and involved in shuttle streaming, the plasmalemma invaginations are found within the outer ectoplasmic wall. Rounded-up parts of this branched extracellular labyrinth limit the endoplasmic core engaged in the mass transport of protoplasm by shuttle streaming. Despite this clearcut borderline, the central endoplasmic core and the ectoplasmic cortex are connected by occasional protoplasmic bridges. The extracellular phase within the ectoplasmic regions of the strands can be interpreted either as a result of plasmalemma invaginations fiom the outer border of the strand, or as a consequence of pseudopodial-like processes originating from the central core and extending into the surrounding medium. The invagination system provides an extensive enlargement of the surface area within the multinucleate protoplasmic mass, probably important for food absorption, excretion processes and motility phenomena. In thick protoplasmic strands with diameters between 0·2 and 15 mm, there is an intimate connexion between the actomyosin fibrils and the invagination system. The fibrils are attached to the plasmalemma invaginations and/or run parallel to the invaginated plasmalemma sheets. The close relations between the invagination system and actomyosin fibrils will be described in detail in a subsequent paper.