RESPIRATION AND OXYGEN. 87 



Summary. The general rule that growth is decreased or prevented 

 by the absence of oxygen appears to suffer but a single exception. 

 Jaccard stated that rarefied air brought about an acceleration of 

 growth accompanied by modifications in aspect, but it seems prob- 

 able that this was due to elongation and did not involve an actual 

 increase in dry weight. Saussure found that most plants and plant 

 organs died in a vacuum in the absence of sunlight, and explained 

 the exceptional behavior of marsh-plants as due to the large amount 

 of oxygen in their tissues. Crocker and Davis have shown that 

 seedlings of Alisma grow considerably in vacua, but that this is little 

 more than 50 per cent of their aerobic growth. Saussure, Jentys, 

 Cannon, and Cannon and Free have found pure nitrogen to stop 

 growth quickly or to greatly retard it, except in marsh-plants, and 

 helium has much the same effect. Detmer and Palladin obtained 

 no growth with seedlings in hydrogen, and Wilson noted that a 

 mixture of nineteen-twentieths hydrogen and one-twentieth air pro- 

 duced a marked decrease, while Jentys likewise found this gas 

 unfavorable to growth. Bert, Wieler, Jentys, Lehmann, and Crocker 

 and Davis have all demonstrated that reduced air-pressure retards 

 or inhibits growth, and Nabokich and Cannon have obtained the 

 same results with amounts of oxygen of 2 per cent and below. 



While pure oxygen or air-pressures of several atmospheres have 

 generally been shown to retard growth, Bert, Jentys, and Jaccard 

 have all observed exceptions to the rule. It is certain that both 

 the species and the time of exposure account for very wide variations 

 in response. Carbon dioxid naturally produces the same effect upon 

 growth that it does upon germination, and all investigators are in 

 accord in finding it very injurious. As a rule, the amount required 

 to cause injury in mesophytes ranges from 5 to 10 per cent, but in 

 hydrophytes and some xerophytes especially, it may be as high as 

 20 per cent, and, in exceptional cases, even higher. 



The question of the possibility of the growth of phanerogams in 

 the continued absence of oxygen has been discussed chiefly by Wieler, 

 Nabokich, Godlewski and Polzeniusz, and Lehmann. While Nabo- 

 kich has maintained that growth can regularly occur in the absence 

 of oxygen, he is forced to admit that it differs from normal aerobic 

 growth in its dependence upon sugar solutions and upon temperature, 

 and in the universal death of the cells in oxygen-free media. His 

 own experiments are hardly free from criticism, as he has taken no 

 account of the air contained in the seedlings at the outset, which 

 might well be enough to meet the exceedingly low requirements of 

 the sunflower, as shown by Wieler. Moreover, it would seem diffi- 

 cult to prove that the small amounts of growth observed were not 

 due to imbibition or turgor forces, as suggested long ago by Saussure. 

 Finally, the most significant result was the death of 63 per cent of 

 the seedlings at the end of a period of 34 to 36 hours without oxygen. 



