1)6 BOTANICAL GAZETTE [September 



**Diese An- und Abschwellung resultirt aus dem periodisch 

 schwankenden Wassergehalt der Theile; dieser ist regelmassig 

 am Tage geringer, in der Nacht hoher." **Der ungleiche Was- 

 sergehalt der Organe bei Tag und Nacht ist seinerseits das Resul- 

 tat aus der ungleichen Thatigkeit der Wasser zu- und abfiihrenden 

 Krafte/' We will return later to the tensions associated with the 

 water content. The rarefaction of the gas in the tracheae is 

 often spoken of as causing a negative tension. The physical 

 condition of a gas makes it impossible that it should exert or 

 even transmit a real negative tension. A gas must always press 

 against whatever confines it; when the gas is rarefied the pres- 

 sure diminishes, but it can never become a suction. 



While lateral movement of w^ater in the wood is difficult, its 

 passage lengthwise of the wood is exceedingly easy. This is 

 easily and conclusively shown by what is known as the '*Th. 

 Hartig experiment." As an instructive experiment, we owe this 

 to Sachs; but no one experiment can be identified as ** Sachs' 

 experiment," so the misnomer can profitably be retained. As a 

 matter of historical interest it may be remarked that the experi- 

 ment did not originate with Hartig. Boucherie (1841) had 

 made a technical application of it in substantially the same 

 improved form afterward employed by Sachs. And Biot [cf, Biot, 

 1841) preceded Boucherie, but I have not had access to his ear- 

 lier papers. Hartig showed that when water was forced out of 

 a stick by warming it, gravity could determine the place of its 

 appearance. Sachs' familiar experiment is to soak a stick in 

 water, dry the ends, place a drop on the upper end, and observe 

 that it is absorbed, while a corresponding drop 'emerges below. 

 Various woods make good subjects for this experiment, those 

 containing vessels allowing the freest movement. As inter- 

 preted by Godlewski (1882 : 585-7), it shows that the weight 

 of the water held in the wood is sufficient to overcome the resist- 

 ance to its movement. Schwendener (1886: 579; also Strasbur- 

 ger, 1891:773) showed that much less than this pressure is 

 required. Strasburger also showed that an excess pressure of 

 only 20 per cent, is required to cause a movement in sticks or 



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