142 AERATION AND AIR-CONTENT. 



or to the water-retaining power of peat, advanced by Warming, 

 Friih and Schroter, Davis, and Burns. Low temperature has gen- 

 erally been recognized as a primary factor in boreal, polar, and alpine 

 bogs, following the conclusions of Kihlmann, Schimper, Goebel, 

 Warming, Meigen, Friih and Schroter, Transeau, Burns, and others. 

 This is confirmed from another direction by Gates's demonstration 

 that the bog evergreens are winter xerophytes. 



The oxygen-content of the soil has been regarded as the control- 

 ling factor by Hesselmann, Clements, and Bergman, and as a pri- 

 mary factor by Warming, Friih and Schroter, Whitford, Transeau, 

 Coville, and Burns. The importance of toxic compounds, which is 

 an outcome of the earlier view that humic acids were a factor, was 

 first suggested by Livingston, and has been advocated chiefly by 

 Dachnowski and by Rigg. Finally, Clements has maintained that 

 most so-called bog xerophytes are not xerophytes at all, but hydro- 

 phytes or rarely mesophytes that owe their peculiar impress to the 

 stability of ancestral characters. Similar views as to the significance 

 of fixed characters were early advanced by Warming for certain 

 sedges, by Schwendener for xeroid sedges and grasses, and by Sten- 

 strom for the evergreen bog plants. 



The work of Clements, Sampson and Allen, Gates, Otis, Folsom, 

 Dosdall, Bergman, and Clements and Goldsmith has shown that a 

 large number of the so-called bog xerophytes are hydrophytes. It 

 clearly indicates that the greater number of the bog xerophytes listed 

 by Kihlmann and by Warming will prove to be hydrophytes when 

 their water-relations are determined. In fact, it appears probable 

 that the term "bog xerophyte" will finally be restricted to the broad- 

 leaved evergreen ericads and similar plants, whose xerophily has been 

 shown by Gates to be due to winter rather than to the direct action of 

 bog conditions. The species of supposed bog xerophytes as listed 

 by Warming and others that have been shown to be hydrophytes are 

 the following: Sagittaria latifolia, Ranunculus sceleratus, Scirpus 

 lacustris, Carex filiformis, Pontederia cordata, Scirpus americanus, 

 Typha latifolia, T. angustifolia, and Equisetum fluviatile. The results 

 now being obtained in continuation of this study indicate that prac- 

 tically all helophytic sedges, grasses, rushes, alismals, thin-leaved 

 dicotyl herbs, and deciduous shrubs will prove to be hydroj)hytic in 

 their water relations. 



The universal need of oxygen by flowering plants, the nature and 

 products of anaerobic respiration, the regular presence of air-passages 

 or aerenchyma in amphibious and floating hydrophytes, as well as 

 of stomata constantly open, all indicate that the lack of oxygen is the 

 controlling factor in swamps and bogs, and the presence of toxic 

 substances a consequence of the resulting anaerobic respiration. In 

 this, all organisms that demand oxygen, and produce alcohol, organic 

 acids, or other deleterious substances in its absence, have a share, 



