512 APPENDIX 



With a little practice it becomes possible to detect differences 

 of O-i in the value of the Pg. 



For more accurate determinations, electro-metric methods 

 are employed, for the details of which special textbooks must 

 be consulted. 



The application of these principles has not only given to 

 biochemical problems a much greater precision but have led 

 to the recognition of new facts in the organism itself and to a 

 new factor in its environment. To take a few examples for 

 illustration : Gustafson * found a gradient in the hydrogen 

 ion concentration, varying with age, in a number of plants, 

 the direction of which varied in different individuals of the 

 same species, and in the same organs of different species. 

 Thus the older leaves of Zea, Phaseolus, and Cucurhita pepo 

 (squash) have a higher hydrogen ion concentration than the 

 young leaves, whilst in Helianthus and Cucurhita pepo (pump- 

 kin) the reverse is the case. In the stems of the maize, sun- 

 flower, and pumpkin, the hydrogen ion concentration increases 

 from the base to the apex. In the instance of the Vigna 

 sinensis, Clevenger f observed a variation in the same organ 

 during the course of a day. With respect to environment, 

 Salisbury % found that the soil of natural woodlands shows 

 a stratification in which a definite gradient of hydrogen ion 

 concentration obtains, the maximum being at the surface. 

 The surface of the soil is poorest in bases which increase in 

 amount with increasing depth, the buffer action being greatest 

 in the layer of maximum organic content. These points are 

 significant in that they play a part in the distribution and 

 density of the bacterial flora. § Interesting and important 

 observations have also been made by Atkins || with regard to 

 the sea, the hydrogen ion concentration of which varies at 

 different places in response to depth, season, and degree of 

 carbon assimilation by the algal plankton : — 



* Gustafson : " Amer. J. Bot.," 1924, 24, 11. 

 t Clevenger : " Soil Sci.," 1919. 8, 227. 



X Salisbury ; " J. Ecol.," 1922, 9, 220. See also Atkins : " Proc. 

 Roy. Dublin Soc," 1922, 16, 369. 



§ See Thornton : " Ann. Appl. Biol.," 1922, 9, 241. 



II Atkins : " J. Marine Biol. Assoc," 1922, 12, 717 ; 1923-5. '3» 93- 437- 



