228 



Papers from the Department of Marine Biology. 



absorbed 2.8 c.c. per hour, showing that 4.7 c.e. per hour was produced 

 by photosynthesis, at 30°. In other words, the O2 given out in the day 

 is about two-thirds the amount used at night. 



The production of oxygen by plants by photosynthesis depends on a 

 number of factors. The data found in the Hterature are all in terms of 

 leaf surface and not volume or weight. Haberlandt found the capacity 

 of the chloroplasts of different plants to be about the same — i. e., the 

 rate of photosynthesis per unit area depended on the number of chloro- 

 plasts per unit area, as shown in table 9. 



Table 9. 



With the same plant, photosynthesis depends on temperature, light, 

 and CO2 supply, and a lowering of any one of these factors may make it 

 the limiting factor (law of minimum) . Blackman showed that photo- 

 synthesis is about doubled by a rise of 10° in temperature. He showed 

 this to be true of temperatures as high as 45°, provided the rate immedi- 

 ately after raising the temperature was calculated. With injurious 

 temperatures the rate constantly falls, and the initial rate was estimated 

 by extrapolation of the time curve. 



Blackman and Smith, working on fresh- water plants, showed that 

 photosynthesis is about doubled by a rise of 10° in temperature and 

 varies directly with CO2 tension in the water and illumination. Their 

 maximal values, reduced to 30°, were about as follows, in cubic centi- 

 meters CO2 per square centimeter per hour: Fontinalis 0.149, Elodea 

 0.093, Ceratophyllum 0.245. 



Most of the determinations were made with Elodea and some leaves 

 overlapped, so that the average thickness was a little more than that 

 of one leaf. If we assume a thickness of 0.1 mm. and a specific gravity 

 of unity, 9,300 c.c. of CO2 per kilogram per hour were consumed in 

 photosynthesis (about 10 per cent of this was re-formed by respiration 

 of the plant during the day and the same amount during the night, so 

 that perhaps 9,000 c.c. was converted into oxygen for the use of animals) . 

 This is a maximum value and would be reduced in case of marine plants 

 by decrease in light due to clouds, inclination of the sun, and absorption 

 by the water. A very rapid use of CO2 by seaweed causes a crust of 

 CaCOs to form on the surface, thus keeping out light and CO2. 



