Direct evidence is presented in support of the widely held idea that membrane-bounded vesicles can bind firmly to microtubules. This is shown in P. caudatum which contains ribbons of straight microtubules located in open cytoplasm and uniquely associated with the disk-shaped vesicles. These vesicles frequently lie flat against the face of the ribbons at a constant distance of 30-40 nm. Under certain conditions the ribbons are compressed into zigzag pattern, but the vesicles continue to maintain their 30-40 nm spacing with the tubules and The author's interpretation of this phenomena is that the vesicles and the microtubules are strongly bound together. This interaction appears to be via a filamentous material rather than bridges.

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