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CARNEGIE INSTITUTION OF WASHINGTON. 



as yet been made of the CO2/O2 content of uncultivated soils in arid 

 regions, there is reason for supposing that this may vary considerably 

 from season to season, and especially in soils of different fineness. 

 This conclusion rests in part on the known great differences in the 

 texture of soils in the arid regions, and in part on differences in root- 

 oxygen carbon-dioxide responses as exhibited by xerophytic perennials 

 of unhke root-habits. 



It is presumable that the surface soils in the arid regions during dry 

 seasons are all adequately aerated, but this is not the case during the 

 rainy seasons. The soils of finest texture — for example, adobe — 

 become easily saturated with water and in the puddled condition free 

 air-movement is impossible. Under the same conditions of rainfall 

 the other and coarser soils either are not puddled, or if so, the saturated 

 condition lasts for a relatively short time only. The importance of 

 the distinction of soils according to their texture lies in the fact that 

 in all soils, but significantly so in those of the finest texture, the 

 season of greatest vegetative activity occurs in midsumimer, when 

 such soils are puddled. It therefore seems possible, owing to known 

 slowness of movement of gases by diffusion only, that the oxygen- 

 content of saturated soil at this time may be quickly and considerably 

 lowered, while at the same time the amount of carbon dioxide may be 

 noticeably increased. With the passing of the rainy season this con- 

 dition of poor soil aeration would naturally cease. 



Inadequate soil aeration works in two ways. The accumulation of 

 carbon dioxide, through its known narcotic effect, operates to depress 

 growth and also, as the amount of available oxygen becomes less, 

 growth for this reason diminishes in intensity. It is probable that 

 there is a specific response to parallel amounts of CO2/O2 in the soil, 

 as shown by land plants of unlike root-habit, especially. It has already 

 been shown (Carnegie Inst. Wash. Year Book No. 15, 1916) that 

 excessive amounts of carbon dioxide are more toxic to roots of Opuntia 

 versicolor than to roots of Prosopis velutina. Recent experiments have 

 shown that the roots of these two species have an unlike relation to 

 a diminished amount of oxygen in the soil air. This is indicated by 

 the following summary. In these experiments parallel series of cul- 

 tures were grown at constant temperatures and in soil. The summary 

 gives the observed length of time in days that root-growth was seen 

 to occur in the two species and growing under similar conditions: 



