THE NATURE OF PHOTOSYNTHESIS 179 



Photosynthesis . ,. „ 



— — 7 — -. for ditferent temperatures: 



Respiration 



20 — 22° 0.5882 0.4227 0.4280 



2 — 3.5° 1.603 0.9207 2.059 



Plaetzer found, that while the different species vary greatly in the 

 light intensity at which compensation is attained, the same species seems 

 to possess a definite comi^ensation point. Harder,^^^ on the other hand, 

 was able to demonstrate that the compensation point is a variable quantity, 

 varying more than 100 per cent, for any one species depending upon 

 the previous conditions of illumination of the plant. Thus, ivy leaves 

 which had grown in the direct sunlight showed a compensation point of 

 2477 Lux, while similar leaves, grown in the shade, one of 1133 Lux. 

 Harder was able, moreover, to demonstrate that with continued dark- 

 ness the compensation point is reduced although the difference between 

 the two sets of plants, sun and shade plants, is maintained. Thus, two 

 sets of Fontinalis antipyrctica, the one taken from a sunny, the other 

 from a shady habitat, possessed, after corresponding periods in darkness 

 the following compensation points : 



Date ■ Sun Plants Shade Plants 



Sept. 20 152 Lux 95 Lux 



26 118 79 



30 Ill 74 



Oct. 2 84 64 



5 59 < 41 



10 27 about 10 



Harder also showed that by starting with the same culture of Clado- 

 pliora and keeping one portion in diffuse light and another in direct 

 sunlight great differences in the compensation point result within seven 

 days. 



Similar results have been obtained by Boysen-Jensen."^ In Shiapis 

 alba, a light plant, the compensation point lies at 1.0 (Bunsen units X 

 100) ; while in Oxalis acetosella, a shade plant, the compensation point lies 

 at 0.2. 



It is evident, therefore, that the compensation point is by no means of 

 constant value for any single species of plant and that it is essential that 

 the previous condition of the plant as to illumination must be taken into 

 consideration before attempting to draw conclusions from the value of 

 the compensation point. Harder's results show that under circumstances 

 in Fontinalis photosynthesis more than overbalances respiration at an 

 illumination of 10 Lux, while under other circumstances 150 Lux is not 

 sufficient to accomplish this. To show that the compensation point can 



Harder, Ber. hot. Ges.. 41, 194 (1923). 



Boysen-Jensen, Bot. Tidsskr., 36, 219 (1918). Staelfelt, "Meddel. fran Statens 

 Skogsforsoksanst.," Stockholm, 18, No. 5 (1921). 



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