DEPARTMENT OF BOTANICAL RESEARCH. 61 



species tolerant of a large amount of carbon dioxid are also relatively, 

 or actually, insensitive to a total or partial deprivation of oxygen, such 

 generalization needs support from actual experimental studies with the 

 particular species in question. Also, whether the converse is true 

 remains to be shown, but it has been learned, in unreported studies, 

 that in the case of Prosopis velutina and Opuntia versicolor, at any rate, 

 the tolerance to an excessive amount of carbon dioxid in the atmosphere 

 of the soil of the two species is apparently about the same, but the 

 former is less injuriously affected by a diminution in the oxygen supply 

 per se. To use oxygen, therefore, in connection with carbon dioxid 

 alone in attempts to define the aeration relation of roots is apparently 

 not justifiable. 



Anaerobic Experiments with Helium, hij W. A. Cannon and E. E. Free} 



In the annual report of this Department for the year 1919 mention 

 was made of experiments on deficient soil aeration, in which experi- 

 ments hehmn was used as the diluting gas instead of the nitrogen or 

 hydrogen commonly employed. The continuation of these experi- 

 ments has confirmed the observation that nitrogen and heUum do not 

 behave exactly similarly when used as the inert gas in soil-aeration 

 experiments. No difference is observed between the two gases when the 

 conditions are entirely anaerobic, and no difference is observed when the 

 gas mixtures contain an ample percentage of oxygen. The differences 

 are encountered with nitrogen and helium, each containing small per- 

 centages of oxygen. 



For instance, in order that the root of the common garden pea shall 

 be able to gi'ow successfully, it is necessary that the soil atmosphere 

 contain at least 1.5 per cent of oxygen, the remainder of the atmosphere 

 being nitrogen. However, when the soil atmosphere consists mainly of 

 helium, a content of less than 0.5 per cent of oxygen is sufficient for the 

 growth of the root. Similar differences have been observed in the case 

 of the germination of peas, the production of chlorophyll by rice 

 seedlings, the phototropic curvature of the seedlings of the sunflower, 

 the diurnal movement of acacia and oxalis, the stigmatal movement 

 of Diplacus glutinosus, and several other plant activities. In all cases 

 the difference is of the same character, namely, that a smaller percent- 

 age of oxygen is required for the progress of the given plant activity 

 when the diluting gas is helium than when the diluting gas is nitrogen. 



It is beheved that this difference is due to the fact that oxygen 

 diffuses through helium more rapidly than through nitrogen. How- 

 ever, since the differences are observed not only with roots growing in 

 the soil, but also with germinating seeds, Diplacus flowers, and other 

 Uving material exposed to a free atmosphere, it is necessary to assume 

 that the dift^erence in diffusion of oxygen will be effective even in the 



^ Research Associate of the Department for six months. 



