Some Experimental and Theoretical Observations Concerning Mass Flow in the Vascular Bundles ofHeracleum

Abstract

The theory and practice of applying the thermodynamics of irreversible processes to mass-flow theories is presented. Onsager coefficients were measured on cut and uncut phloem and cut xylem strands of Heracleum muntegazzimum. In 0.3 N sucrose + 1 mN KC1 they are as follows. In phloem, L_EE = 5 × 10–⁴ mho cm⁻¹, L_pE = 9 × 10⁻⁶ cm³ s⁻¹ cm⁻² volt⁻¹ cm, and L_PP = 0.16 cm³ s⁻¹ cm⁻² (J cm⁻³)⁻¹ cm. In uncut phloem strands L_EE is about 1 × 10⁻³ mho cm⁻¹. In xylem in 2 x 10⁻³ N KCI, L_pp = 50 to 225, L_PE = 2 × 10⁻⁴, and L^EE = 4 × 10⁻³. The measurements are tentative since the blockage of the sieve plates is an interfering factor, but if they are valid they lead to the conclusion that neither a pressure-flow nor an electro-kinetic mechanism envisaging a ‘long distance’ current pathway can be the major motive ‘force’ for transport in mature phloem. Measurements of biopotentials along conducting but laterally detached phloem bundles of Heracleum suggest, nevertheless, that there may be a small electro-osmotic component of at least 0.1 mV cm⁻¹ endogenous in the phloem.