Cohesion of Water in Plants.

Forces applied tlhrough the cohesion of wvater have been generally accepte(d as the cause of the rise of the transpirationi streaml since the early publications of Dixon and( Joly (8). The water absorbing forces of the leaf mlesophyll cells caln be shown to be adequate to lift w!ater to the height of the leaves. The cohesion theorv assumiies that the (liffusion of water from the non-collapsible xyleml elements in contact with these leaf cells creates a state of tension within interconnectedl water columns in the xylem. This tension is possible because of the cohesion of water molecules an(l their adhesion to the hvdrophilic walls of the xxylelem elements. Tensioni in the water columns is assumie(l to lift water fronm the roots to the leaves and, in addition, to re(luce the free energy of the water in the root xylem until wvater (liffuses from the soil into the root during passive absorption of water. Greeni(dge (9) ancd Hillebrandl (13) have discusse(l briefly the various metlhods of estimating the internal pressure an(d potential cohesion of water. These give v-alues varying from about 4.000 to nearly 40.000 atmospheres. They are estimates, however, rathel thaii nmeasurements and it is not established that cohesion is equivalent to internal pressure. The first successful attempt to measure cohesion in wN-ater experimentally seems to have been made by Berthelot (2) in 1850. He obtaine(d values to 50 atmosplheres by a mlethod similar to that used later by Dixon anid Jolv (8). The results obtained by the Dublin workers wvere stumminiarize(d by Dixon (7) in 1914. Estimates of colhesion 1b (listol-tion of glass tubes were about eight atmospheres while those basedl on the Berthelot metlhod wNere as high as 200 atmospheres. Ursprung (26) calculatedl that tensions on the ordler of 300 atlmiospheres were reaclhedl in the annulus cells of discharging fern sporaingia, and King (15) colncluded from an analysis of the (lvnamics of the fer-ni alnlnulus that cohesion valuies greater than 10 an(l less than 10,000 atmospheres w-ere concerned. Direct measurements of cohesion (1 24). in contrast, give maximiium values of onlyr 2 or 3 atmospheres, enough to demlonstrate the reality of the process, but not to lift the transpiration streamii in tall trees. A numtiber of recent workel-s have raised further (luestions concerning the functioni of the cohesion of w-aterin plants: much of this w-ork is (liscussed by