  We show that the formation of membrane tubes (or membrane tethers), which is
a crucial step in many biological processes, is highly non-trivial and involves
first order shape transitions. The force exerted by an emerging tube is a
non-monotonic function of its length. We point out that tubes attract each
other, which eventually leads to their coalescence. We also show that detached
tubes behave like semiflexible filaments with a rather short persistence
length. We suggest that these properties play an important role in the
formation and structure of tubular organelles.
