80 AERATION AND AIR-CONTENT. 



with a twelfth 583 mg. In the shade, the smallest addition of carbon 

 dioxid proved injurious. The plants died in 6 days in a fourth, and 

 in 10 days in a twelfth. A superabundance of C0 2 was more in- 

 jurious to a plant in nitrogen than in ordinary air, for, while a twelfth 

 of carbon dioxid in ordinary air was not injurious to marsh-plants, 

 they died in a few days with such an amount in nitrogen. 



John (1819) showed that plants which were brought into an at- 

 mosphere of 33 to 50 per cent of CO 2 or more developed poorly. 

 Davy (1821 : 205) also observed that plants showed but poor growth 

 in air that contained one-third to one-half of its volume of CO 2 . 

 Birner and Lucanus (1866 : 160) endeavored to determine the effect 

 of CO 2 in a nutrient solution on growth. Carbon dioxid was led into 

 the solution every 2 or 3 days, but an unfavorable effect was observed 

 in neither of the two plants. On the contrary, they developed ex- 

 cellently and were much stronger than those not acted upon by C0 2 . 

 The conclusion was reached that the action of free C0 2 is favorable 

 to the production of dry weight, though it was thought that this 

 was due to the actual use of the gas by the leaves, either by absorp- 

 tion through the roots or by diffusion through the air. 



Later researches. Bert (1873, 1878) made a comprehensive study 

 of growth in reduced air-pressure and in reduced oxygen-content, as 

 well as under increased air-pressure and increased oxygen-content. 

 He was able to show that air-pressure as such had no influence upon 

 growth, but that the latter was influenced only by the partial pres- 

 sure of the oxygen. Studies with the germination of corn, barley, 

 and cress showed that the process took longer in a pressure of 50 to 

 25 cm. than with a normal air-pressure, and that germination finally 

 ceased at a pressure of 8 to 7 cm. In the case of Mimosa pudica it 

 was found that the leaves fell and the plant quickly perished under 

 a pressure of 25 cm. Algae were found to cease growth at a similar 

 pressure. 



Bohm (1873 : 141) found that the development of seedlings in 

 pure oxygen was usually restricted to the first stage in germination, 

 and concluded that the injury was not due to the lack of nitrogen, 

 but to the density of the oxygen itself. In general, the growth of 

 plants from a supply of reserve material was as a rule reduced to a 

 minimum in pure oxygen of ordinary density. However, growth took 

 place with the same intensity as in atmospheric air when the oxygen 

 was so rarefied by the air-pump or the admixture of hydrogen that 

 it possessed a pressure equal to or even smaller than the partial 

 pressure of atmospheric oxygen. 



Rischawi (1876) repeated the experiments of Bohm, but he did not 

 find the development of bean, pea, and corn seedlings so near the 

 minimum as Bohm, although it was weaker than in ordinary air. 

 The most important result was that the rootlets of the plants in 



