98 AERATION AND AIR-CONTENT. 



are able to grow aerobically under certain conditions. In fact, 

 Beijerinck (1897) has called in question the existence of permanent 

 anaerobes. 



The bacteria, yeasts, and molds that produce fermentation are 

 the commonest of temporary anaerobes, since their ability to use 

 sugar enables them to dispense with free oxygen for a time (Smith, 

 1895). They differ greatly in their ability to hve without oxygen, 

 Ascomyces growing better than in the air, and some of the yeasts 

 reproducing repeatedly in its absence, while the bread-mold soon 

 ceases growth and becomes inactive. Fermentation may continue 

 until stopped by the accumulation of alcohol, while growth ceases 

 more quickly and reproduction still earlier. 



Practically all fungi are affected by carbon dioxid, but to a much 

 smaller degree than chlorophyllous plants. While a few bacteria 

 and yeasts may grow more or less normally in pure carbon dioxid, 

 the growth of the great majority is retarded or inhibited, and many 

 are quickly killed by it. The common molds, Mucor, Aspergillus, and 

 Penicillium, were unable to germinate in pure carbon dioxid, though 

 the spores were not killed by an exposure of 3 months. In Mucor spore 

 formation was stopped by 20 per cent CO 2, growth stopped at 30 to 

 40 per cent, and germination at 60 per cent, while for Aspergillus and 

 Penicillium the respective percentages were 50, 80, and 90. 



While the importance of the bacteria in the soil has been univer- 

 sally recognized, much less attention has been given to the molds and 

 other fungi found in the soil. Waksman (1916) has recently sum- 

 marized the investigations in this field and has given an account of 

 his own studies (1917). He has isolated over 200 species of fungi 

 from 25 different soils in North America and the Hawaiian Islands, 

 representing the Mucor acece, Sphceriacece, Mucedinacece, Demaiiacece, 

 Tuber culariacecB, and Saccharomycetacece. It appears certain that 

 molds must play an important part in many soils, and especially 

 in those with deficient aeration. While too httle is known of their 

 products under anaerobic conditions, there is no question that they 

 produce organic acids and other substances that must be taken into 

 account in connection with acidity and toxicity. 



AERATION AS AN ECOLOGICAL FACTOR. 



Sorauer (1873, 1886, 1895, 1909) was perhaps the first to thor- 

 oughly appreciate the importance of aeration in practice, doubtless 

 because he has been the chief exponent of the non-parasitic diseases 

 of plants. In the successive editions of his "Handbuch der Planz- 

 enkrankheiten" he has dealt with the effects of deficient aeration in 

 detail. These are discussed under various captions, namely, lack 

 of oxygen (1909 : 312), puddling of the soil (190), flooding and for- 

 swamping (195), souring of seeds and potted plants (201), excessive 



