A method is described which permits measurement of sap pressure in the xylem of vascular plants. As long predicted, sap pressures during transpiration are normally negative, ranging from -4 or -5 atmospheres in a damp forest to -80 atmospheres in the desert. Mangroves and other halophytes maintain at all times a sap pressure of -35 to -60 atmospheres. Mistletoes have greater suction than their hosts, usually by 10 to 20 atmospheres. Diurnal cycles of 10 to 20 atmospheres are common. In tall conifers there is a hydrostatic pressure gradient that closely corresponds to the height and seems surprisingly little influenced by the intensity of transpiration. Sap extruded from the xylem by gas pressure on the leaves is practically pure water. At zero turgor this procedure gives a linear relation between the intracellular concentration and the tension of the xylem.

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A replicative form isolated by ) from E. coli infected with the RNA phage fr shows the same melting profile, Tm, RNAase resistance, and buoyant density in CS2SO4 as the replicative form of MS2. Although the replicative form of phage fr, purified by the method A of these authors, also sediments with an s20 of about 8.5, their improved method B yielded a preparation containing a minor component with an s20 of 14.5 S. This is the value expected for a duplex containing twice as much RNA as the viral strand. A similar value, of 12-16 S, was obtained by Fenwick et al. (ref. 9) for material tentatively identified as the replicative form of the RNA phage R17 by sucrose gradient analysis of infected E. coli extracts.

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