70 



Michx., Bclclia Raf. for Downingia Torr., Ptiloria Raf. for 

 Stcphanomeria Nutt., etc. In these changes from the usage of 

 the "Botany of California," there is no suggestion of the fifty- 

 year limit proposed by the Berlin botanists and there is little 

 evidence of mercy toward names which, according to some 

 writers, have become so consecrated by long usage as to be out 

 of the reach of" modern nomenclatural legislation. Yet several 

 generic names equally vulnerable, like Capsella Medic, Echino- 

 cystis T. & G., and Dicentra Bernh., are retained. But these 

 possibly await modification in the second edition, which the mani- 

 fest merits and popular qualities of the work will doubtless soon 

 make a necessity. [M. A. H.] 



CORRESPONDENCE 



"A Simple Dynamometer" 



The discussion of this particular apparatus would not deserve 

 more space, did not the criticism involve a principle applicable to 

 a number of instruments for measuring the force exerted by 

 plants. In his reply * to my former letter, Dr. Richards implies 

 that I misunderstood his experiment ; rather, I think, he has 

 missed the point of my objection. I had no thought of criticiz- 

 ing his device because it does not measure the force of imbibition 

 in all directions. The difficulty is that the proposed dynamom- 

 eter does not register correctly any component of the force of 

 swelling, for the simple reason that the spring scale is not adapted 

 to do it. Gravitation can act through an indefinite distance and 

 the weight in the pan descends until the distortion of the spring 

 is as great as the force acting can produce. In swelling, on the 

 contrary, the force to be measured acts through a very limited 

 distance only, and when the limit of its thrust is reached the in- 

 dex stops, whether it indicates an ounce or a ton. The principle 

 is that distortion of a system, however registered, can never be 

 used to measure correctly any force, unless the possible distor- 

 tion is greater than that necessary to produce the maximum 

 registration of the instrument. 



The caution regarding overloading, therefore, is not pertinent, 



*Torreya, i : 48. Ap. 1901. 



