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SCIENCE 



[N. S. Vol. XJjV. No. 1156 



these objects shall be obtained we may 

 easily estimate both our national intelli- 

 gence and our peace-loving qualities. 



Must it be the grim necessity of war only 

 that shall awaken us to a nation-wide 

 mobilization of all human forces for a single 

 great purpose? Have we the mental ca- 

 pacity to be led by reason rather than 

 driven by necessity "to sacrifice comforts, 

 indulgences, and elegancies" for the pur- 

 pose of acquiring the knowledge of self 

 without which a practical preparation for 

 living is impossible 1 



Stewaet Paton 



Princeton, N. J. 



THE ECOLOGICAL SIGNIFICANCE OF 

 SOIL AERATION 



During the past two years the writers have 

 conducted, independently, laboratory investi- 

 gations into the relation of plant roots to the 

 composition of the soil atmosphere and espe- 

 cially to deficiency of oxygen or excess of car- 

 bon dioxide in this atmosphere. These inves- 

 tigations are still in progress and will be re- 

 ported later. It seems, however, that certain 

 features of the results already obtained have 

 important ecological significance, and this 

 phase of the matter is presented in the present 

 preliminary paper. 



One series of experiments was conducted by 

 one of us (Cannon) in the Desert Laboratory 

 of the Carnegie Institution of Washington at 

 Tucson, Arizona, and in the Coastal Labo- 

 ratory of the same institution at Carmel, Cali- 

 fornia. Seedlings of Prosopis velutina, and 

 cuttings of Opuntia versicolor were grown in 

 glass tubes filled with sand and connected with 

 a gas reservoir in such a manner that any de- 

 sired gas or mixture of gases could be caused 

 to replace the ordinary atmosphere of the tubes 

 at will. Each tube was sealed with wax and 

 usually a water seal was used in addition. By 

 the use of appropriate thermostats the tubes 

 were kept at any desired temperature. In all 

 cases the shoots were exposed to the atmos- 

 pheric conditions of the laboratory. The 

 growth of individual roots was observed di- 



rectly by means of a horizontal microscope. 

 The experiments included tests with pure 

 carbon dioxide and various mixtures of this 

 gas with atmospheric air or with oxygen. 



As a leading result it was learned that the 

 roots of Prosopis and of Opuntia have unlike 

 responses to carbon dioxide. Exposure to pure 

 carbon dioxide causes cessation of growth in 

 the roots of both species. However, the re- 

 covery on the admission of air is uniformly 

 more rapid with Prosopis than with Opuntia. 

 The two species respond differently, also, to 

 percentages of carbon dioxide which are high 

 but below 100 per cent. Thus mixtures of 

 50-75 per cent, carbon dioxide with 25-50 per 

 cent, oxygen, do not stop root growth of Pro- 

 sopis, but do stop that of Opuntia. Appar- 

 ently excessive amounts of carbon dioxide in 

 the soil atmosphere would act as a limiting 

 factor for the latter plant, even if the oxygen 

 content of the soil atmosphere was normal or 

 above normal. ITeither excess of carbon diox- 

 ide nor diminished supply of oxygen inhibits 

 the growth of Prosopis roots, for growth did not 

 wholly cease when an atmosphere containing 

 less than 2 per cent, of oxygen was employed. 

 However, entire deprivation of oxygen appears 

 to inhibit growth since the roots did not grow 

 in pure carbon dioxide. Thus while the effects 

 of the undiluted carbon dioxide on root growth 

 of the two species is apparently the same, 

 namely, the cessation of growth, the responses 

 leading to this effect may be quite different. 



The conclusion that the root response to a 

 variable ratio of oxygen to carbon dioxide in 

 the soil atmosphere is a specific response, is 

 supiKirted further by the results of direct 

 aeration experiments on several species of 

 plants, among which were Opuntia, Prosopis, 

 Fouquieria splendens, and garden varieties of 

 cucumber and watermelon. An increased air 

 supply to the roots of Opuntia and Fouquieria, 

 if not excessive, favors root branching and 

 probably accelerates the rate of root growth. 

 In the case of Prosopis, increased aeration of 

 the soil appeared not to affect the growth rate 

 of the roots. The results with cucumber and 

 watermelon were not consistent, although in 

 the latter case the shoot growth appeared to be 



