86 CARNEGIE INSTITUTION OF WASHINGTON. 



cedure is open to the objection that nitrogen is known to react with 

 numerous other substances and with certain Uving organisms, this 

 suggesting a possibiUty that soil atmospheres of nitrogen may not be 

 entirely inert as regards plant roots, soil bacteria, and other living 

 elements of the soil. The recent development of processes for the 

 procurement of helium, and the cooperation of the Bureau of Steam 

 Engineering of the U. S. Navy in supplying a quantity of this gas, 

 have made it possible to repeat previous experiments, using helium 

 instead of nitrogen. Because of its absolute inertness chemically, it is 

 highly improbable that this gas could have any specific or direct 

 effect on any constituent of the soil. Preliminary tests with mixtures 

 of the helium and air have shown the gas used to be free of any impurity 

 harmful to ordinary plants. 



Experiments on the garden sweet pea have been made, using the 

 technique previously described by Cannon, in which the seedlings are 

 grown in sand wetted with nutrient solution in glass tubes at constant 

 temperatures and the rate of root-growth has been observed directly. 

 Three tubes were used. In one the normal soil-atmosphere was replaced 

 by nitrogen; in the second, helium was used instead of nitrogen; in the 

 third, a slow stream of helium was maintained through the tube. In 

 the tubes containing static atmospheres of helium and nitrogen respec- 

 tively, root-growth stopped at once. In the tube wdth the stream of 

 helium, root-growth continued at a rate not greatly, if at all, below the 

 normal growth in air. After three days air was again supplied to 

 the roots. In all cases root-growth recommenced, but in the case of 

 the plant exposed to nitrogen the main root did not grow, although 

 the laterals did. In the other cases (after helium) both the main roots 

 and the laterals grew. 



Experiments with the ordinary potato were made according to the 

 technique described by Livingston and Free, in which the plant-roots 

 and the soil in which they grow are sealed into an air-tight tin can, the 

 plant-stem passing through the seal. On repeated experiments wdth 

 potato plants, both with and without tubers, the injury to the plant 

 is evident substantially sooner when the normal soil-atmosphere is 

 replaced by nitrogen than when helium is used. Examination of the 

 roots shows the injury to consist in a softening and decay of all proto- 

 plasmic and storage tissues, in which the tuber, when present, is also 

 involved. This decay is similar in character in cultures with nitrogen 

 and helium, but progresses more rapidly in the former gas. 



A comparative series of cultures, in which the carbon dioxid pro- 

 duced in the soil was measured, led to results summarized in the fol- 

 lowing table. Six cultures were included; two had potato plants with 

 tubers, two had plants without tubers, and two had soil only. In one 

 of each sort the soil-atmosphere was replaced by helium, in the other 



