AN AUTOMATIC TRANSPIRATION SCALE OF LARGE CA- 

 PACITY FOR USE WITH FREELY EXPOSED PLANTS 



By Lyman J. Briggs, Biophysicist in Charge, Biophysical Investigations, and H. L. 

 Shantz, Plant Physiologist, Alkali and Drought Resistant Plant Investigations, 

 Bureau of Plant Industry. 



INTRODUCTION 



An extended study of the transpiration rate of plants practically neces- 

 sitates the use of an automatic balance of some type. The present paper 

 contains a review of the various forms of transpiration balances hereto- 

 fore employed, together with a description of a new automatic transpira- 

 tion scale of large capacity, so designed that the plants may be freely 

 exposed to the weather. Four of these scales have been in continuous 

 use during the past four summers at Akron, Colo. 



Automatic balances may be divided into two classes: (i) The step- 

 by-step type, in which small weights of equal value are added to the 

 scale pan in succession or a counterpoise is advanced in equal steps; (2) 

 the continuous record type, in which the plant is suspended from a spring 

 or from a variable lever or is mounted directly on a float. 



RECORDING BALANCES OF THE STEP-BY-STEP TYPE 



Vesque (1878)* appears to have been the first to employ an automatic 

 balance in measuring transpiration. He made use of the step-by-step 

 principle, a measured quantity of mercury being delivered to a recep- 

 tacle on the scale pan each time the beam tipped sufficiently to close an 

 electric circuit. His apparatus is illustrated in figure i, the device for 

 measuring the mercury being shown at s and enlarged at B. This meas- 

 uring device is in principle similar to a large stopcock, in which the plug 

 is only partially bored through from each side so as to form two shallow 

 cavities of equa^l volume. Either cavity in its upper position becomes 

 filled with mercury from the reservoir t. When the circuit is closed, a 

 spring motor is released, which turns the plug through one-half a revolu- 

 tion, delivering the mercury in the cavity to the container a, and record- 

 ing the time of the event by lowering the stylus p in contact with the 

 circular plate v of the clock H. 



Anderson (1894) was the first to employ steel balls of uniform size as 

 weights for a recording balance. The balls were held in a spiral brass 

 tube, with a block at the lower end containing a pocket for one ball. 

 When the balance beam tipped sufficiently to close an electric circuit, 

 the block was moved sidewise and the ball in the pocket dropped into the 



1 BiblioKraphic citations in parentheses refer to " Literature cited," p. 1 51-132. 



Journal of Agricultural Research, Vol. V, No. 3 



Dept. of Agriculture, Washington, D. C. Oct. 18, 1915 



aj G-S9 



(117) 



