22 AERATION AND AIR-CONTENT. 



in an atmosphere with insufficient oxygen to meet the demands of 

 rapid respiration. Experiments proved that the only conditions 

 necessary for the production of blackheart were excessive tempera- 

 tures and insufficient oxygen. 



Stewart and Mix (1917 : 277) found that no sprouts start and any 

 already present blacken and die when jars full of potatoes are sealed 

 at 70 F. Moisture appears on the surface of tubers after 10 days 

 and the condition of blackheart develops in the interior. The 

 amount of air for the proper maintenance of tubers was determined by 

 two series of experiments. When the number of volumes of air 

 ranged from 4.6 to 9.5 per volume of potatoes, sprouts barely 

 started and were dead at the end of 40 days. From 10.5 to 14.7 

 volumes they were very feeble, and were likewise dead at the end. 

 With 15.7 to 18.5 volumes the growth was feeble and the sprouts 

 dead, while at 19.7 to 33.2 volumes the sprouts were normal. In 

 the one series examined for it, blackheart was present from 4.6 to 

 22.2 volumes. 



Coville (1910) has emphasized the importance of good aeration 

 for the swamp blueberry and other heaths. The ideal condition of 

 the peat about the roots is one of constant water-content during the 

 growing-season, but with such drainage that thorough aeration is 

 secured. The high degree of aeration obtained is thought to explain 

 the success attending the use of coarse kalmia peat, since pure peat 

 was not used by the earlier heath-growers. Without understanding 

 the importance of air, they secured aeration by mixing pieces of 

 pots or sandstone with the soil. 



Shull (1911 : 476) decided that the seeds of Xanthium can not 

 grow without comparatively large amounts of oxygen, while the 

 naked embryos of the two seeds require very different amounts of 

 oxygen for germination. The delay in germination is largely due 

 to the nature of the coats which exclude the oxygen. In a later 

 study (1914 : 64) he was able to show that the seed-coats of seeds 

 of Xanthium glabratum used 20 per cent as much oxygen as the seeds 

 themselves. The respiration of the lower seeds was greater than 

 that of the upper in a ratio of 1.35 : 1. The lower seeds with coats 

 off used from 2 to 5 times as much oxygen as those with the coats on. 

 Babcock (1912 : 111) found that, while intramolecular respira- 

 tion might occur in seeds containing more than 10 per cent of water, 

 germination never took place except when direct respiration was pos- 

 sible. Free oxygen was essential, though it was not always neces- 

 sarily present in the gaseous state. Some seeds in water utilize 

 dissolved oxygen, but only those of water plants can grow properly 

 under these conditions. A number of seeds of cultivated plants de- 

 composed hydrogen peroxid rapidly, and then germinated as readily 

 as in the air. 



