The Work Performed in Transpiration and the Resistance of Stems 



By Carlton C. Curtis 



It is a matter of some difficulty to make a clear demonstration 

 of the amount of work performed by a plant in transpiration. The 

 usual method of measuring the extent of this important operation, 

 t. e., through the lifting power of the transpiration current, is open 

 to objection, since the stem usually allows the air to infiltrate and 

 stop the rise of the column of mercury long before the measure of 

 the force of transpiration has been reached. 



A nearer approach to the extent of this force may be attained 

 by subjecting a given length of the stem to pressure and forcing 

 water through it at a rate equalling that of the transpiration 

 stream. This may be easily accomplished, after determining the 

 rate of transpiration with a Darwin potometer, by removing the 

 capillary tube from the potometer and attaching it with a short 

 piece of rubber tubing, about 20 cm., to one end of the stem, while 

 the other end of the stem is connected with a burette by means of 

 a longer length of tubing, so as to allow wide latitude in the 

 amount of pressure applied. The tubings are filled with water 

 before attachment to the stem, and care must be exercised to ex- 

 clude the entrance of any air, since the latter acts as an effectual 

 block to the passage of water. These connections are effected 

 under water with more ease and safety from introduction of air. 

 For short lengths of stem an apparatus stand will give sufficient 

 elevation to the burette to furnish the necessary pressure. 



The burette and tubings having been properly attached, any 

 water that passes though the stem escapes through the capillary 

 tube. By pinching the tube connected with the capillary tube be- 

 tween the fingers, a given amount of water can be forced out and 

 absorbed with a blotter, and with the removal of the pressure the 

 water in the tube will be drawn back. With a few trials it is pos- 

 sible to judge accurately just how much water to force out in this 

 way, so that the capillary tube will be left nearly empty upon re- 

 leasing the pressure. The water passing through the stem will 



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