The sieve tube wall and its relation to translocation.

The sieve tube wall possesses a broad inner layer often with pronounced radial striations. The plasmalemma of the sieve tube appears to penetrate this wall in the form of a "brush border" of irregular microvilli, greatly increasing its surface area. It is suggested that this is the site of active transport of potassium, which circulates electroosmotically through the sieve plate pores and back through the thick wall. The function of the companion cells is the "care and maintenance" of the active "brush border" sites; in conjunction with their activity in supplying high-energy intermediates movement in the column acts regeneratively and fully polarises the plates. Many of the lamellar stacks and curvilinear membrane aggregates hitherto regarded as endoplasmic reticulum are, it is suggested, plasmalemma displaced from the wall. These findings have important consequences for the electroosmotic theory.