lyo BOTANICAL GAZETTE [September 



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tinue until after 2:00 p. m., even on bright days, as I have observed 

 in B. lenta. Ernst (1876:35) tells of water guttated by Callia^idra 

 Saman falling like a fine rain all day. 



Root pressure rises rapidly with the temperature of the 

 ground. The bubbles of air in the tracheae enlarge when warm, 

 and, as Sachs points out (i860; Hofineister, 1862), this will 

 force water upward and may well work with real root pressure 

 in the spring bleeding of grape, maple, etc. I would hesitate 

 to ascribe so much importance to it, however, as does Stras- 

 burger (1891:833 seq.). If bleeding were very largely due to it 

 the daily periodicity would be reversed, as sometimes occurs, 

 especially near the close of the bleeding season (Hartig, 

 1863:277 ; Clark, /. c). 



According to Wieler ( 1893:63) guttation is dependent on the 

 presence of free oxygen. Wieler construes this as an argument 

 that r.oot pressure is caused by differences in concentration mam- 

 tained within the cell. But Purjewitsch^s ( 1897)^ discovery of 

 the great influence of oxygen on diffusion through the proto- 

 plasm converts Wieler's point into a better one for the otherwise 

 more plausible theory that root pressure depends on permeability 

 to the solute. 



The minor role played by root pressure in the ascent of sap 

 is obvious from the limited work possible by any pressure to be 

 observed, and from the fact that it is altogether absent at the 

 times during the season when transpiration is most active. On 

 the latter point enough has been said. As to the pressures, 

 those which I have cited are the highest ev^er recorded, with the 

 best subjects growing in our climate. With ordinary subjects 

 which show any root pressure at all it is usually only a few 

 centimeters, or only millimeters, of mercury. It is true that j* 



the pressure as measured by manometers is an excess over the 

 counter pressure of the atmosphere. Root pressure unable to 

 overcome this can be demonstrated in many plants (Scheit 

 1886:701 ; Wieler, 1893:97 seq.) if the atmospheric pressure is 



^From instances in which the food normally dififuses from the cell without 

 apparent change, I am unable to agree with Purjewitsch in ascribing the effect of the 

 presence or absence of oxygen to its action on enzymes. 



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