RESPIRATION AND OXYGEN. 27 



consequence, there is a complete chain of evidence from Mayow 

 (1668) to the present time as to the necessity of oxygen for root 

 activity. The direct proof of this has been obtained by showing the 

 use of oxygen by roots, and this has been confirmed again and again 

 by their behavior in the absence of oxygen. It is a fact significant 

 of the influence of the nature philosophers that the researches of 

 Ingenhousz, Senebier, Rollo, Huber, and Saussure were followed 

 by an almost barren period of 50 years. In spite of the experiments 

 of Grischow, Boussingault, and a few others the labors of the earlier 

 investigators were largely lost sight of, and their results were in a 

 large degree to be gained anew. 



The direct evidence of the necessity of oxygen for the proper 

 functioning of roots has been furnished by Ingenhousz, Senebier, 

 Saussure, Grischow, Garreau, Aeroboe, Vochting, Kossowitch, Snow, 

 Raciborski, Brizi, Day, Hunter, Hall, Brenchley and Underwood, 

 Harrison and Aiyer, Hole and Singh, Howard and Howard, Cannon, 

 Livingston and Free, Bergmann, and Schley. In addition to a vast 

 amount of evidence derived from studies of germination and anaero- 

 bic respiration, the respiratory behavior of tubers, bulbs, and other 

 underground parts affords further confirmation of that of roots. 

 Such studies have been made chiefly upon the potato since the pio- 

 neer work of Nobbe (1865), and the investigators concerned have 

 been Kny, Bohm, Miiller-Thurgau, Mangin, Ziegenbein, Jost, Rich- 

 ards, Strohmer, Vochting, Smirnoff, Griiss, Miiller and Schneider- 

 Orelli, Iraklionow, Appleman, Bartholomew, Hasselbring and Haw- 

 kins, Stewart and Mix, and Butler. 



The excretion of carbon dioxid by roots was first noted by Hales 

 (1727) and was confirmed by Ingenhousz, Saussure, Garreau, and 

 Corenwinder, as well as by many recent observers. Its acid nature 

 was first demonstrated by Wiegmann and Polstorf (1842) and by 

 Oudemans and Rauwenhoff (1858). The significance of the excreted 

 C0 2 in the economy of the soil was first suggested by Moldenhawer 

 (1812), but was proved by Sachs's studies of the etching power 

 exerted by roots upon rock (1860-1864), and confirmed by Knop 

 (1861, 1864) and Cauvet (1880). Recently Fred and Haas (1919) 

 have returned to this point to show that the presence of bacteria 

 greatly increases the etching power of roots. Sachs had thought 

 it possible that other organic acids might have a part in corrosion 

 phenomena, but Stoklasa and Ernest (1905, 1908) have shown that 

 carbonic acid alone is concerned in this under conditions that permit 

 normal respiration. 



Molisch (1887) concluded from his experiments that root excretions 

 have both an oxidizing and reducing action, as well as a weak dia- 

 static one. Czapek (1896) was unable to confirm the presence of a 

 ferment, but he found that in addition to carbon dioxid, roots 

 secreted considerable quantities of monopotassium phosphate, which 



