900 TRANSACTIONS OF SECTION K. 



critically considered but few points. I Lave ratlier endeavoured to distribute 

 imperfect data in the perspective in which they appear from the point of view of 

 one who seeks to simplify phenomena by extending the principles of chemical 

 mechanics as far as possible into the domain of vital metabolism. Much critical 

 quantitative work has yet to be done before the whole becomes an intelligible 

 picture. 



To me it seems impossible to avoid regarding the fundamental processes of 

 anabolism, katabolism, and growth as slow chemical reactions catalytically accele- 

 rated by protoplasm and inevitably accelerated by temperature. This soon follows 

 if we once admit that the atoms and molecules concerned possess the same 

 essential properties during their brief sojourn in the living nexus as they do before 

 and after. 



Perhaps the more real question is rather as to the importance and significance 

 of this point of view. Protoplasmic activity might be something so much per se, 

 and the other factors of the nature of stimuli might be superposed so thickly 

 upon that substratum which should be dominated by simple principles of chemical 

 mechanics that for practical purposes the operations of the latter would be so 

 overlaid and masked as to be negligible. A survey of this field, however, seems 

 to show that this is not so, and that the broad action of the law of mass and the 

 acceleration of reaction-velocity by temperature are obviously responsible for 

 wide ranges of phenomena. 



Now the conception at the bottom of these principles is that of reaction- 

 velocity, and the conclusion of the whole matter is that the physiologist must 

 frankly take over from physical chemistry this fundamental conception.' Under 

 definite conditions of supply of material and temperature there is a definite 

 reaction-velocity for a given protoplasm, and the main factors that alter the rate of 

 metabolism, viz., heat, nutrition, and traces of impurities are exactly the factors 

 which affect the velocity of reactions in vitro. 



"Working on this basis we no longer need the vague unquantitative terminology 

 of stimulation for the most fundamental of the observed ' responses ' to external 

 conditions. Three sets of phenomena we have observed which, though usually 

 treated in the category of stimulation, draw a clearer interpretation from the 

 conception of reaction-velocity. These were: (1) the relation of development to 

 the absence or deficit of single essential food constituents; ('2) the occasional 

 striking efiect of minute traces of added foreign substances uj)ou the whole rate of 

 growth and metabolism ; and (.3) the general doubling of the activity of vital 

 processes by a rise of 10° C. 



The next higher stratum of principles should be the complications introduced . 

 by limiting factors which interrupt the extent of the manifestations of these 

 principles and by various correlations, as, for example, that by which the reaction- 

 velocity of one catabolic process might withdraw the supply of material needed 

 for full activity of another different process. To this sort of relation may be 

 attributed that phenomenon so characteristic of the more complex vital processes 

 and quite unknown in the inorganic world, namely, the optimum. 



Finally, superposed upon all this comes the first category of phenomena that 

 we are content still to regard as stimulatory. From the point of view of meta- 

 bolism and reaction-velocity many of these appear very trivial, though their 

 biological importance may be immense. Think how little'the tropistic curvatures 

 of stems and roots affect our quantitative survey; yet a little rearrangement of 

 the distribution of growth on the two sides of an organ may make the difference 

 between success and failure, between life and death. 



From our present point of view vision does not extend to the misty concep- 



' No general treatment of the physiology of jlants has yet been attempted in 

 terms of reaction-velocity. Czapek, however, in the introduction to his stupendous 

 Biochemle der Pfanzen, vol. i. 190.o, does draw attention to the conception of 

 'reaction-velocity' aud refer to the standard literature on this subject and on 

 catalysis, though direct application is not made to the plant. Cohen {Physical 

 Chemistry for Physicians and Biologists, English edition, 1903) considers in detail 

 some biological applications of the acceleration of reactions by temrerature. 



