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THE AMERICAN NATURALIST [Vol. XL 



to form in wood of Taxus at -10° or -11° C. Therefore only 

 during the coldest nights of the sugar season could ice ever form 

 in the wood. Some of the aberrations in the readings obtained 

 by various investigators on very cold nights seem to be due to this 

 cause, since it is not improbable that if ice actually does form in 

 the vessels an increase of pressure at night rather than a decrease 

 may be evident for a time. 



Freezing, therefore, plays no important part in the phenomena 

 under discussion. 



Theory that Pressure is due to Activity of Living Cells 



Having exhausted the possibilities in which mere physical force 

 due to expansion is the main factor, we come now to the theories 

 in which protoplasm plays the main part. 



Pressure cannot be caused by the contraction of the protoplasm 

 with forcible ejection of the sap because of the fragile nature of 

 the ectoplasm. Even if this were sufficiently strong no pressure 

 could be obtained unless the exit from the contracting sack was 

 into a reservoir unconnected with the space around the remaining 

 surface of the sack. Otherwise the extruded sap would simply 

 occupy the space left by the contracting protoplasm, and no in- 

 crease in volume would take place. The alternative then, is for 

 the pressure to be caused by osmotic phenomena. Pressure and 

 flow if accounted for in this way must presuppose an exudation, 

 under pressure, of sap from the living cells. 



Exudation is known to occur in Mucor as described by Pfeffer. 

 In Spirog}Ta at very low temperatures near zero C, water has been 

 observed to appear in droplets upon the surface of the cells. ^ 



Drops of water are secreted from the cells of the pulvinus in 

 Mimosa when stimulated, and from the sensitive staminal filament 

 of the Cynarete. At present bleeding pressures in root and stem 

 tissues can be accounted for in no other way. The phenomenon 



> Pfeffer. Pflanzenphysiologie, ed. 2. 



Greeley, A. W. "On the Analogy between the Effects of Loss 

 and Lowering of Temperature." Amer. Joum. Physiol., vol. 6, p. 



Livingston, B. E. The Role of Diffusion and Osmotic Pressure 

 Chicago, 1903. 



