CYTOPLASMIC POLARITY AND ENDOPLASMIC MICROTUBULES ASSOCIATED WITH THE NUCLEUS AND ORGANELLES ARE UBIQUITOUS FEATURES OF FOOD-CONDUCTING CELLS IN BRYOID MOSSES (BRYOPHYTA)

An ultrastructural survey of food conducting cells in the gametophytes and sporophytes of bryoid mosses, including the highly specialized leptoids of Polytrichales, has revealed that, without exception, these cells have a polarized organization and contain an axial system of endoplasmic microtubules (MTs). The orientation of polarity is highly consistent with the putative direction of the source to sink gradient, with distal cellular ends, i.e. those toward the sink, containing denser cytoplasm than proximal ends. In most cases the nucleus is spindle-shaped and also lies at the distal end. Longitudinal arrays of MTs are associated with the nuclear envelope. These MTs extend into the cytoplasm for long distances and may come into contact with the plasma membrane lining the terminal walls at the distal ends. Cortical MTs are absent. The endoplasmic MTs are closely associated with a variety of organelles including mitochondria, plastids, endoplasmic reticulum and membrane-bounded tubules and vesicles. The organelles aligned along the MT bundles lie within longitudinal cytoplasmic strands clearly visible with light microscopy. The discovery of cytoplasmic polarity and endoplasmic MTs as distinctive features of food conducting cells of mosses sets these cells apart from the sieve elements of vascular plants. The MT-organelle associations, as observed in the food conducting cells of mosses, are hitherto undescribed in plants but are reminiscent of MT arrays functioning in organelle transport in neuronal axons and other animal and fungal systems.