68 AERATION AND AIR-CONTENT. 



were more or less injurious. His experiments lasted but a short 

 time and gave no light upon the effect of long-continued exposure. 



Schiitzenberger and Quinquand (1873) found that the maximum 

 photosynthesis in Elodea was at 5 to 10 per cent of CO 2 dissolved in 

 water; it was reduced by 20 to 30 per cent, and ceased in saturated 

 CO2. 



Warburg (1886 : 122) found that the differences in photosynthesis 

 for Nerium were not very significant at 5 to 25 per cent, while in 

 Bryo'phyllum, photosynthesis was slight at 12 per cent and minimum 

 at 20 per cent. Hoplophyturn grande showed a marked reduction at 

 10 to 15 per cent. 



Pfeffer (1887) likewise observed that photosynthesis was suppressed 

 in the absence of oxygen. Correns (1892) confirmed the conclusion 

 of Wiesner that oxygen is necessary for the greening of etiolated 

 plants. This occurred at 4 per cent for Helianthus, 5 per cent for 

 Sinapis, and 8 per cent for Lepidium. 



Brown and Escombe (1902 : 397) determined that a leaf responds 

 to increased carbon dioxid in the air around it within certain limits. 

 With 2 to 4 times as much CO2 in the air, the activity of photosyn- 

 thesis was increased, but the gain in dry weight was less than in 

 ordinary air. The effect upon the plant was shown by shorter, 

 thicker internodes, a more bushy habit due to the development of 

 axillary buds, smaller leaves, and the almost complete suppression 

 of flowers. They concluded that the sudden increase of CO2 in the 

 atmosphere to 2 to 3 times the present amount would destroy nearly 

 all flowering plants. 



Crocker and Davis (1914) determined that no chlorophyll de- 

 veloped in seedhngs of Alisma during a month's exposure to an air- 

 pressure below 5 mm. 



Summary. — All results are in accord as to the harmful effect of 

 carbon dioxid upon photosynthesis and the production of chlorophyll. 

 The latter function appears the more susceptible, as Bohm observed 

 that injury was produced at 2 per cent, but an extensive study of 

 this relation in a wide range of plants is much to be desired. Bohm 

 also determined that several hours' exposure to an atmosphere de- 

 void of oxygen was sufficient to destroy the power of photosynthesis, 

 and Boussingault obtained similar results from the use of carbon 

 dioxid, nitrogen, or marsh-gas for 48 hours. Grischow found that 

 pure carbon dioxid greatly reduced but did not wholly prevent 

 photosj^nthesis in the sunlight, and Boussingault stated that, while 

 it was slight in the pure gas, it was marked at 30 and even 40 per 

 cent. These percentages are much higher than those obtained by 

 other investigators, and it seems probable that they were due to the 

 dilution of the gas by the oxygen freed in the sunlight. 



The varying response of different species is shown by the fact 

 that Pfeffer discerned no effect from 8 and 16 per cent of carbon 



