RESPIRATION AND OXYGEN. 45 



fall, porous soils have a larger air-content than fine or compact ones, 

 and this is true even when they approach saturation, owing to the 

 readier movement of water in them. Water-logging is practically 

 confined to heavy soils, and swamps are typical of alluvium. Satu- 

 rated soils contain air only in solution, and the amount available is 

 sufficient only for water-plants, which possess some device for aera- 

 tion in the case of helophytes and plotophytes. As many investi- 

 gators have shown, land-plants, with the rarest exceptions, fail to 

 find enough oxygen in saturated or water-logged soils, and die as a 

 consequence. 



As shown in a following section, the amount of carbon dioxid 

 necessary to produce injury ranges from 2 to 20 per cent, depending 

 upon the plant. Such percentages are far from infrequent, especially 

 in manured and water-logged soils. Boussingault and Lewy found 

 9.74 per cent in manured soil wet by frequent rains, while Fleck 

 obtained 7.96 per cent in garden soil, and Smolenski 10.2 per cent 

 in soil that had been contaminated. Mangin determined that 

 amounts of carbon dioxid ranging from 3.89 to 24.84 per cent were 

 sufficient to retard the leafing of Ailanthus, the higher percentages 

 being fatal to the trees. The elm was much more sensitive, retarda- 

 tion occurring from 1.71 to 5.81 per cent. Harrison and Aiyer found 

 maxima ranging from 6.6 to 21 per cent in the gas of cropped and 

 manured rice plots. Leather obtained maxima of 10.1 and 18.4 per 

 cent in fallow fields after green manuring, and of 12.30, 16.99, and 

 21.14 per cent about the roots of plants. These results make it 

 practically certain that injury from CO2 is much more frequent than 

 is commonly supposed, especially in field and garden soils that have 

 been manured, and indicate that it must be taken into account in 

 all cases of toxic action. 



AIR-CONTENT OF WATER. 



Morren and Morren (1841 : 9) determined the composition of the 

 gas of a vivarium 20 feet in each direction, following the changes 

 throughout the growing-season from March to September. They 

 found that the CO2 varied from 1.27 per cent to 23.04 per cent, and 

 often very rapidly, these two extremes occurring on the 9th and the 

 19th of August. This maximum amount of CO2 led to the death of 

 small animals and finally to that of the fishes. The amount of oxy- 

 gen varied from 18.01 per cent to 60.43 per cent, the close relation 

 to the CO2 being shown by the fact that the maximum occurred 

 on August 9 and the minimum on August 19, the dates for the 

 minimum and maximum of CO2. 



Whipple and Parker (1902 : 104) have given a comprehensive 

 series of tables of oxygen and carbon dioxid in dealing with the effect 

 of gases on microscopic organisms. At 20° C, 1 Hter of water can 

 absorb 28 c.c. of oxygen, 14 c.c. of nitrogen, and 901 c.c. of carbon 



