86 AERATION AND AIR-CONTENT. 



15 mm. per day for the 8 days the roots were subjected to nitrogen. 

 Shoots of potato, on the contrary, stopped growing very soon after 

 the access of nitrogen. Growth was decreased in the case of morning- 

 glory, tomato, and corn. Carbon dioxid in mixtures containing 25, 

 50, and 75 per cent exerted a depressing effect upon root-growth in 

 Krameria canescens, and growth ceased after a time, to be renewed 

 upon the re-entrance of air. In Covillea the effect was very harm- 

 ful, growth being quickly retarded and soon ceasing. It was renewed 

 but slowly in the air. In Mesembryanthemum growth was decreased, 

 but it ceased only after relatively long exposures, and was renewed 

 with relative rapidity. The difference in sensitivity to carbon dioxid 

 was shown by the fact that the root-growth of Covillea ceased in 

 1.5 hours in 25 per cent CO2, that of Krameria in 2.5, and of Mesem- 

 hnjanthemum in over 4 hours. 



Experiments with such low amounts of oxygen as 2 and 1 per cent 

 and with nitrogen (1920 : 60) again demonstrated the varying sensi- 

 bility of different species. With 2 per cent oxygen, root-growth 

 stopped promptly in the case of onion, and was much retarded in 

 most of the other species. However, in the plants of wet soils, 

 *Juncus, Potentilla, and Salix, and in the succulent Mesembryanthe- 

 mum, as well as in alfalfa, growth was decreased but slightly. In 1 

 per cent oxygen, growth ceased quickly, though in the species of 

 wet soils and in Mesembryanthemum growth continued for several 

 days. In commercial nitrogen root-growth continued for over 5 

 days in Mesembryanthemum, over 7 days in Juncus and Potentilla, 

 and for 13 days in rice, while it was inhibited in the other species. 

 Cannon and Free (1919 : 86) have shown that the root-growth of 

 the sweet-pea stopped at once in static atmospheres of nitrogen or 

 helium, but continued for 3 days in a stream of helium at a rate 

 little below normal growth in air. Injury to potato roots took place 

 soonef in nitrogen than in helium. In later experiments (1920 : 61), 

 this difference in the action of nitrogen and helium was confirmed, 

 though it does not occur when conditions are entirely anaerobic or 

 when an ample amount of oxygen is present. With nitrogen, 1.5 per 

 cent of oxygen is required for the root-growth of the garden pea, but 

 with helium 0.5 per cent suffices. Similar differences have been found 

 in the germination of peas, the greening of rice seedlings, leaf move- 

 ments in acacia and oxalis, stigma movements of Diplacus glutinosus, 

 etc. It is suggested that this difference is due to the fact that 

 oxygen diffuses more rapidly through helium than through nitrogen. 

 Kidd (1919 : 248) has shown that 5 to 10 per cent of oxygen is the 

 optimal amount for the sprouting of potato tubers. Above this it 

 is harmful, while 80 per cent inhibits it in 4 to 5 weeks. Its injuri- 

 ous effect is increased in the presence of carbon dioxid. The latter 

 inhibits sprouting at 20 per cent, and higher concentrations cause 

 marked injury and death. 



