1902] RISE OF THE TRANSPIRATION STREAM 1S9 



>erfol2:te in dcr Reorel bei kleinen Reichbeblatterten Pflanzen 



^ ^ *" """" -^"^^^ 



schneller als bei grosseren wenig beblatterten/' etc. Bohm's 

 conclusion (562), *'Dass die zum Saftsteigen erforderliche Kraft 

 von dem Luftdrucke geliefert werde/' is equivalent to saying 

 that an actually negative tension cannot be exerted to absorb 

 the water. Vesque (1884), working with sound water cultures of 

 oleander, found that the absorption became impossible when the 

 tension was one atmosphere minus 63''"' Hg. Strasburger 

 {1891:793), Scheit (1886, II), and Janse (1887:3-10) all agree 

 that dicotyledons wilt if water is available under too low a 

 pressure, and that the removal of nearly the whole atmospheric 

 pressure always makes it too low. Saussure (p. 30 of Ostwald's 

 Klassiker no. 16, p. 214 of original) describes a somewhat 

 different experiment explicable in the same way; vessels contain- 

 ing plants w^ith their roots in water were placed in v actio 2.x\6. lived 

 for weeks, and even grew, if protected from direct illumination. 

 Under the conditions the plant was in an atmosphere saturated 

 with water and did not transpire. Water may have diffused 

 rapidly enough to supply the demands of what growth occurred 

 in a ''vacuum/' In direct light the plants dried up, because the 

 leaves became warmer than the air, and water evaporated from 

 them and could not be replaced. 



We have found not only that negative tensions do not occur 

 in dicotyledons in nature, but that an artificial elimination of 

 the positive tensions to which they are accustomed robs them of 

 the power to absorb and elevate the transpiration stream. There 



priori 



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theory which, for the sake of thoroughness, we will still con- 

 sider. Berthelot says (p. 332-3) ** A ce moment (when negative 

 tension has been caused bv coolinof) le moindre choc ou broisse- 

 ment, la moindre vibration fait reparaitre a I'instant, avec une 

 sorte d'ebullition, un leger bruit et une secousse plus ou moins 

 notable, le gaz dissous dans I'eau." Lehmann (p. 244), speaking 

 of water under a tension of one atmosphere minus 1,200 °"" Hg, 

 says " Erst bei erschutterunoren volQ^t in solchen Fallen das 

 Sinken zum normalen Standpunkt." Donny seems to be 



