47 



CORRESPONDENCE 



"A simple Dynamometer" 



In the first number of Torreya, Dr. H. M. Richards describes 

 briefly " a very simple machine for registering approximately the 

 amount of energy involved" in imbibition.* Passing over the 

 terms here used (to which the physicist would take serious ex- 

 ception), it is obvious that the force of imbibition cannot be meas- 

 ured by the arrangement described. Dr. Richards has apparently 

 confused the force of imbibition with the extent of swelling. The 

 attraction in virtue of which water is imbibed, being probably 

 molecular or analogous thereto, is not dependent on the number 

 of organized structures (such as cell-walls) involved, but the ex- 

 tent of the swelling is. The scale in the arrangement suggested 

 registers not force but distance in terms of weight. To illustrate : 

 If the bottle contained only one layer of peas the scale might 

 register a quarter of an ounce, since the distance through which 

 the pan would be depressed might equal the depression which 

 that weight would cause in the particular spring used (a weak 

 one). If the bottle were nearly filled, however, and the peas did 

 not jam but moved freely upward as they swelled, the scale might 

 register half a pound. Yet the actual force of imbibition in the 

 two cases would be exactly the same, and vastly greater thari 

 either registration. Evidently also the result would be wholly 

 different with a very strong spring, an equal depression corre- 

 sponding perhaps to ioo lbs. 



The same objection would lie against the use of the scale for 

 measuring the force exerted in growth. 



It may be worth while, further, to call attention to the fact 

 that a like error inheres in all methods of measuring the force of 

 root-pressure in decapitated plants when a large open tube is 

 used as a manometer. f To a less extent this objection applies 

 also to open mercury manometers. — C. R. Barnes, University of 

 Chicago. 



* Richards, H. M. A simple Dynamometer. Torreya, i : 8. Ja. 1901. 



f Atkinson. Elem. Bot. 32, and Lessons in Botany, 51. Here, regarding a device 

 recommended by Detmer merely to demonstrate the outflow of sap, it is said, " The 

 height of this column of water is a measure of the force exerted by the roots." 



