336 Curtis : The Work Performed in 



cause the water to flow again through the tube, and the current 

 may be timed over the same course as in the transpiration tests. 

 The base of the capillary tube is attached to the rubber tubing in 

 order that the same lapse of time may intervene before measuring 

 the rate as is utilized in the measurements with the potometer. It 

 is important that no more water than the volume of the capil- 

 lary tube should be forced out, otherwise air will be drawn back 

 into the rubber tube and introduce sooner or later troublesome 



j 



While it is feasible to test only short 



lengths of stems by this method, the considerable resistance offered 

 by such pieces gives a very striking and accurate basis for an esti- 

 mate of the work performed in transpiration. The objection might 

 be raised that in subjecting stems to pressure in this way histo- 

 logical elements would become active in the transmission of water 

 that were functionless in transpiration. While it must be con- 

 ceded that all cells may not be equally active or are quite func- 

 tionless, in such stems as may be tested by a potometer, we are 

 dealing with only a few annual zones of growth — frequently but 

 one — and therefore no considerable error is encountered from this 

 source. The demonstration by Strasburger that at least several 

 annual zones assist in transpiration, as may also readily be ob- 

 served in sections of stems that have imbibed colored solutions, 

 renders it possible that under the pressures required for short sec- 

 tions of most woody plants the ordinary conducting cells are alone 

 active in the transmission. In the case of some stems of high re- 

 sistance, noticeably succulents, it is very obvious that a pressure 

 that will force water through the stem at a rate equalling that of 

 transpiration causes a considerable exudation from the extra-xylar 

 tissues of the pith and cortex. In such cases, certainly, the action 

 of these normally feebly transmitting cells would lower somewhat 

 the resistance encountered by the passage of water through the 



xylem. 



I. 



I 



4 



3 4 



Abutilon 48 45 49 10 X .8 Acalypha 13 13 29.5 10 X -5 



46 45 



47 46 



12 13 



13 13-5 



48 47 13 14.5 



48 48 !3 #5 j6 



48 48 13.5 17 



