  Tubular crystals, two-dimensional lattices wrapped into cylindrical
topologies, arise in many contexts, including botany and biofilaments, and in
physical systems such as carbon nanotubes. The geometrical principles of
botanical phyllotaxis, describing the spiral packings on cylinders commonly
found in nature, have found application in all these systems. Several recent
studies have examined defects in tubular crystals associated with crystalline
packings that must accommodate a fixed tube radius. Here, we study the
mechanics of tubular crystals with variable tube radius, with dislocations
interposed between regions of different phyllotactic packings. Unbinding and
separation of dislocation pairs with equal and opposite Burgers vectors allow
the growth of one phyllotactic domain at the expense of another. In particular,
glide separation of dislocations offers a low-energy mode for plastic
deformations of solid tubes in response to external stresses, reconfiguring the
lattice step by step. Through theory and simulation, we examine how the tube's
radius and helicity affects, and is in turn altered by, the mechanics of
dislocation glide. We also discuss how a sufficiently strong bending rigidity
can alter or arrest the deformations of tubes with small radii.
