70 PLANT PHYSIOLOGY 



ment, but should try at least two, preferably more, and if possi- 

 ble ten, and should take the mean of the results. It will also 

 be very advantageous for him to arrange with his fellow students 

 to use uniform source and treatment of material throughout, 

 so that the results may be compared and averaged to a single 

 record which will have some value as a standard of comparison 

 for individual results. And he should be constantly on the 

 search for the detection and elimination of the sources of error 

 (page 14). This entire study is well worthy the best efforts 

 of the student, partly because of the clearness with which it 

 illustrates this kind of physiological relations, and partly because 

 of the unusually favorable opportunity it offers for training in 

 the application of precise physical measurements to a physio- 

 logical phenomenon. 



The foregoing experiment, with its clear exhibition of a 

 minimum point of no movement, a rise towards an optimum 

 point of highest movement, and a fall to a maximum point of 

 no movement, all under a steadily increasing temperature, affords 

 an ideal example of this second kind of relation between Proto- 

 plasm and external forces. And the principle -is the same even 

 in those cases in which the force acts only to reduce movement. 

 Such knowledge as we possess seems to show that the forces 

 produce the results not directly or immediately, but indirectly 

 or mediately, by promoting or hindering some of the dominant 

 physico-chemical processes going on in the Protoplasm. Here 

 the forces, present in moderate amount, act mediately or indi- 

 rectly, the results are proportional to their intensities though 

 only within certain limits, and the sum-total of the interactions 

 of them all is the minimum-optimum-maximum result in the 

 Protoplasm as a whole. Theoretically and primarily the results 

 are independent of advantage or disadvantage to the organism, 

 but in fact organisms are so adjusted to the more common of 

 these forces, that the forces act to keep them in an advantageous 

 condition of tone, whence the relation is designated as tonic. 



As noted above, the three cardinal points, minimum, opti- 

 mum, and maximum, are very characteristic of this tonic rela- 



