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.

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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Suimmary. Freezing poilnt depression in xylem sap of mangroves was founid to range from 0.05 to 0.50. in desert plants from 0.01 to 0.16°. In crush juices from leaves of Batis and Salicornlia 9)0 % or more of the freezing point depression iwas made up of sodium and chlorine ionls; in mangroves they constituted 50 to 70 %, the rest probably being organic solutes. Plants growing in seawater have -30 to -60 atmiospheres pressure in the xylem sap. As showin earlier, at zero turgor pressure the intracellular freezing point of the parenchyma cells matches closely the negative pressure in the xylem sap. This agrees with the present data, that the fluid wNhich exu,des from the xylem by applying gas pressure on the leaves is pracitically pure water; freezing point is rarely above 0.01 to 0.020. To perform this ultrafiltration, the plasma membrane is subjected to a hydrostatic pressure gradient which in some cases may exceed 100 atmospheres.It has been shownl that the xylem sap of a variety of mangroves is low in salt. Some species, like Rhizophora, LaguclcXiaria and Sontneratia, exclude salts almost completely. Others, like Avicennia, Aegialitis and Aegiccras, take in 10 to 20 % of the NaCl present in the seaw-ater and excrete it by special glands on the leaves. In Aegialitis the secretion under oil ranged from about 2 % NaCl at night to about 5 % in the day (8), and as much as 7 % has been measured in Avicelnnia niitida. Crush juices from leaves have a high chloride concenitrationl, and both cryoscopic anid plasmolytic studies have showin that the total osmotic potential as a rule exceededl that of the seawvater( 1, 10). WN'heni leaves of these and oaf a variety of other plants were dehydrated to zero turgor pressuire, it was foulndl that the intracellular freezing point closely matclhed the negative hldrostatic pressure of the xylenm sap. Platnts growing in seawater always had a xvlemi sap pressure of at least -30 atmii, and usuallv -40 to -50 atm.