60 CARNEGIE INSTITUTION OF WASHINGTON. 



cent oxygen, 1 per cent oxygen, 0.5 per cent oxygen, balance nitrogen, 

 and nitrogen only. 



The roots of the different species studied showed, as regards the rate 

 of growth, a considerable diversity in reaction to a soil atmosphere 

 containing about 2 per cent oxygen. In the case of onion sets root- 

 growth ceased promptly and in most of the other species it was slowed 

 to a marked degree. However, in Juncus, Mesembryanthemum, Po- 

 tentilla, and Salix, and to a less degree in alfalfa, root-growth was seen 

 to be little if any less rapid than in normal soil atmosphere. The 

 growth-rate in the pea decreased sooner than in the barley, in which 

 the results were to a degree inconsistent. In all instances where a 

 diminished oxygen-supply directly affects the growth-rate, such effects 

 are less marked when the gas is renewed daily, or passed continuously 

 through the culture, than when it is static. 



In a soil-atmosphere containing 1 per cent oxygen, root-growth in 

 most species ceased shortly, if not promptly, after the administration of 

 the gas. In Juncus, Mesembryanthemum, Potentilla, Salix, and in 

 barley, how^ever, a soil atmosphere of this composition did not appear 

 to have an immediate effect on the rate of root-growth. In Juncus a 

 fairly active rate of growth went on for 7 days, during which time the 

 roots were continuously in 1 per cent oxygen. In the case of barley, 

 however, the results were not wholly consistent, in that there were seen 

 variations in the rate of growth as between different plants and in the 

 same plant on different days, which were apparently not to be traced 

 to the effects of the gas alone. 



In a soil-atmosphere of commercial nitrogen only, root-growth 

 continued for over 5 days in Mesembryanthemum and for more than 7 

 days in Potentilla and Salix; it was seen to continue for 13 days in rice. 

 The effect on the rate of root-growth in barley was not observed. In 

 many of the species studied, therefore, including alfalfa, the growth of 

 roots was inhibited by a soil atmosphere consisting of commercial 

 nitrogen alone. Finally, in Juncus root-growth was observed to con- 

 tinue for more than 7 days in oxygen-free soil atmosphere. 



The results above summarized, therefore, confirm and extend those 

 previously reached to the effect that different species may react 

 differently and characteristically to a soil atmosphere consisting of 

 nitrogen only, or of nitrogen and oxygen in kno^vn but subnormal 

 amount. It has also been shown that different species may hold 

 unlike relations to a soil atmosphere rich in carbon dioxid, whether 

 the normal amount of oxygen is present or not. 



From these results it appears that the rate of root-growth may be 

 directly affected by at least two features of the composition of the soil- 

 atmosphere, namely, the presence of carbon dioxid in excess or a de- 

 ficiency of oxygen, both of which are of importance as factors in the 

 aeration of the soil. Although it is known that in certain instances 



