RESPIRATION AND OXYGEN. 107 



system of air-passages and diaphragms, often supplemented by super- 

 ficial roots and a marked movement of the transpiration stream. 

 Such indicators are found typically in Equisetum, Juncus, Heleocharis, 

 Scirpus, Alisma, Sagittaria, Sparganium, etc. Air-passages also 

 occur in some bog-plants, but they are little or not at all developed 

 in the shrubby species, such as Vaccinium, Ledum, Andromeda, 

 Kalmia, Empetrum, etc. In most of these the aeration devices are 

 subordinate to those designed to conserve the water-supply during 

 drought, especially in winter (Gates, 1914). Coville (1911, 1913) has 

 emphasized the importance of good aeration for the successful culture 

 of the blueberry, pointing out that this is secured in nature by the 

 superficial roots as well as by their position in hummocks. It is 

 probable also that mycorrhiza plays an important role, partly in 

 increasing the available nitrogen, and partly also perhaps in directly 

 compensating for the deficit in oxygen. 



The other two groups of aeration indicators consist of plants which 

 grow normally in well-aerated soil. Hence they lack special adap- 

 tations for aeration and consequently serve to indicate a lack of 

 oxygen by their growth or distribution. Those which are some- 

 what tolerant of water-logged and poorly aerated soils respond to 

 reduced oxygen-content by decreased growth and reproduction. 

 Intolerant species drop out, and their reduced number or absence 

 serves as an indicator of conditions. 



Summary. — The results of field studies of aeration are in complete 

 agreement with those obtained from physiological investigations as 

 to the basic importance of oxygen for root activity and the injury 

 wrought by the accumulation of carbon dioxid. The detailed sig- 

 nificance of the lack of oxygen and the abundance of carbon dioxid 

 as ecological factors is discussed in connection with bog xerophytes 

 and soil toxins. Here it will suffice to point out that field research 

 has approached the problem of an oxygen deficit from four different 

 angles, and that the results and conclusions are all in essential accord. 

 The agricultural approach has been made by Sorauer, Deh^rain, 

 Wollny, Brizi, Ehrenberg, Balls, Harrison and Aiyer, Howard and 

 Howard, Main, and Allan, and that of forestry by Vonhausen, Bohm, 

 Mangin, Hesselmann, Bernbeck, Hole, Hole and Singh, and Coventry. 

 Pathological considerations have entered into many of the studies, 

 but they have received especial attention at the hands of Sorauer, 

 Hartig, Mangin, Howard, and Graves, while the ecological outlook 

 has been represented by Warming and Clements. Moreover, a 

 large number of the papers in the next two sections have a more or 

 less direct bearing upon the ecological and practical significance of 

 aeration, but are discussed later, owing to their relevance to the 

 special problems concerned. 



