238 CHEMISTRY OF PLANT LIFE 



results from any given series of changes generally tending to 

 reverse the process by which they are proceeding and so to restore 

 the condition of normal equilibrium. 



Finally, the most characteristic difference between the reac- 

 tions which go to make up the vital activities of a living cell and 

 those of the same chemical substances when in inanimate form in 

 the laboratory lies in the presence in the colloidal mass of the accel- 

 erating catalysts known as enzymes, which are produced by the 

 protoplasm itself in some way which is as yet wholly unknown; 

 and which not only add to the possibilities of rapid chemical 

 change which are afforded by the colloidal nature of the material, 

 but also, because of their extreme sensitiveness to minute changes 

 in environmental conditions, serve to govern both the rate and 

 the direction of the individual chemical reactions which constitute 

 the vital activities of the protoplasmic mass. These enzymes are 

 not distributed uniformly through any given cell, or organism, but 

 are localized in different parts of the cell or tissue and so give 

 to its different parts the ability to perform their various dif- 

 ferent functions. 



References 



ATKINS, W. R. G. "Some Recent Researches in Plant Physiology," 328 

 pages, 28 figs., London, 1916. 



CZAPEK, F. "Chemical Phenomena of Life," 152 pages, New York, 1911. 



CZAPEK, F. "Ueber eine Methode zur direkten Bestimmung der Oberflach- 

 enspannung der Plasmahaut von Pflanzen," 86 pages, 3 figs., Jena, 1912. 



HOBER, M. R. "Physikalische Chemie der Zelle und der Gewebe," 671 

 pages, 55 figs., Leipzig, 1911. 



LIVINGSTON, B. E. "The Role of Diffusion and Osmotic Pressure in Plants," 

 149 pages, Chicago, 1903. 



McCLENDON, J. F. "Physical Chemistry of Vital Phenomena," 248 pages, 

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MACDOUGAL, D. T. "Hydration and Growth," Publication No. 297, 

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SPEIGEL, L. trans, by LUEDEKING, C. and BOYLSTON, A. C. "Chemical Con- 

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THOMPSON, D'A. W. "On Growth and Form," 793 pages, 408 figs., Cam- 

 bridge, 1917. 



WILLOWS, R. S. and HATSCHEK, E. "Surface Tension and Surface Energy 

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