178 PHOTOSYNTHESIS 



Plaetzer furthermore showed with CladopJwra that with increasing 

 temperature, the compensation point (i.e. light intensity required to main- 

 tain respiration — photosynthesis at an equiUbrium) rises more rapidly 

 than the rate of respiration, determined in the dark. Thus, with an 

 increase in temperature from 5° to 25° the rate of respiration showed 

 a proportionate rise of 1 : 4.8. The light intensity required to maintain 

 the compensation point over this same range of temperatures showed a 

 corresponding proportion of 1 : 6.69. 



The explanation of this phenomenon is in all probability not a simple 

 one. Aside from the possibility (not yet established positively) that 

 light has a stimulating action on respiration, it would appear that photo- 

 synthesis, under these conditions, did not increase in proportion to the 

 increase in light intensity. Such a direct proportionality could exist only 

 under conditions where light was the limiting factor. Now it is highly 

 probable that with the increasing light intensity a point was reached 

 where light was no longer the limiting factor and temperature played 

 the role. Since it has been found that with high illumination and high 

 carbon dioxide-concentration the temperature coefficient of photosynthe- 

 sis decreases with increasing temperature (4.3 at 5° to 1.6 at 32°) it 

 would be expected that the rate of photosynthesis would not increase at 

 the same rate as respiration with increasing temi>erature. From the re- 

 cent work of Harder, ^^° it is apparent, moreover, that where one factor 

 ceases to be a limiting factor and another factor commences to play this 

 role is not a definite point, but rather that there is a gradual transition 

 from one condition to another. This fact must also be taken into con- 

 sideration in interpreting the phenomenon just described. Finally the 

 relative decrease in the rate of photosynthesis as compared to that of 

 respiration with increasing temperature must also be viewed in the light 

 of the influence of the "time factor.'' The latter, as well as a discussion 

 of limiting factors have been taken up in a previous section. 



The position of the compensation point of a plant in regard to tem- 

 perature is naturally of great importance to the life of the plant and 

 its relation to the environment. The fact that with decreasing tempera- 

 ture respiration decreases more rapidly than photosynthesis has also been 

 recorded by Harder. ^-^ His results indicate that CladopJwra, at low light 

 intensity, has a higher photosynthetic activity at low temperatures than 

 at higher temperatures. Harder points out that conditions must exist in 

 nature where at higher temperatures the plant gains no material through 

 photosynthesis on account of the excessive respiratory activity, while at 

 lower temperatures with the same light intensity nutritive material is 

 formed in the plant. These conditions would, of course, apply particularly 

 to the seas of the polar zones where the light intensity is low. Harder 

 gives the following ratio of : 



=*' Harder, Jahrb. zviss. Bot., 60. 531 (1921). 

 ^'^'^ Harder, Jahrb. wiss. Bot., 56, 281 (1915). 



