P-Protein in sieve elements : I. Ultrastructure after treatment with vinblastine and colchicine.

The ultrastructure of P1 and P2 proteins in the sieve elements of Heracleum mantegazzianum is described. P1-protein tubules are closely associated with stacks of membranes, are often linked by short cross-bridges, and occasionally display a hexagonal packing. Incubation with the alkaloids vinblastine and colchicine had no discernible effects on the ultrastructure of the sieve elements at various stages during differentiation. Evidence for and against any similarities between P1-protein tubules and cytoplasmic microtubules is discussed.