Chloroplast microtubules in young leaves ofSedum rotundifolium

Well-defined tubular inclusions were detected in mesophyll chloroplasts of youngSedum rotundifolium leaves. The size and distinctly uniform arrangement of tubular inclusions were the most noticeable features. The chloroplast usually contained a large inclusion, sometimes extending almost as long as the chloroplast. Such inclusions were built up from microtubules exhibiting aggregates of either large plate-like or paracrystalline structures depending on the section angles. These inclusions were often quite huge, measuring as much as 7.1 μm in length and 2.6 μm in width. The diameter of the microtubule was approximately 25 nm. The microtubule aggregates were non-membrane bounded structures enclosed partly by the thylakoids. The microtubules in the aggregate were all displaying the same definite orientation. Cross-sectional views clearly demonstrated the characteristic hexagonal arrangement within paracrystalline structures. Longitudinal sections of the chloroplast microtubules showed that they were in perpendicular orientation to the chloroplast envelope. They were not connected to these membranes in any case. In general, one microtubular aggregate was seen in each chloroplast section. The role of tubular inclusions in the chloroplasts related to the photosynthetic mode was discussed.