THE NATURE OF PHOTOSYNTHESIS 157 



NaNO.i. sucrose and glycerine isotonic with 0.5 per cent KNO3 showed an 

 equal inhibiting effect. This inhibiting effect increases with concentration. 

 Concentrations which did not cause plasmolysis produced no permanent 

 effects ; that is, the plant regained its original rate when the salt solution 

 was replaced by water. If plasmolysis had occurred (2.5 per cent KNO3) 

 the plant did not recover its original rate when placed in water. Some- 

 what contrary to these results are those of Kny ^"''' who found that 

 Spyrogyra cells which had been in a 40 per cent solution of sucrose for 

 less than an hour, showed evolution of oxygen when placed in a 20 per 

 cent solution and were illuminated. When the concentration of the 

 sucrose solution was gradually increased from 10 to 40 per cent, some 

 cells still showed photosynthesis after 24 hours. There appears to be 

 no doubt, however, that plasmolysis greatly decreases the rate of photo- 

 synthesis, though the activity may not be entirely prevented. 



Fromageot ^^~ concludes from his studies with Ulva lactuca in different 

 concentrations of sea-water that there is a distinct optimal concentration 

 for photosvnthesis and that this corresponds to that of sea-water 

 (A = 1.94)'. 



g. The Time Factor. 



Investigations of the eft'ect of temperature on photosynthetic rates have 

 shown that the maximum rate cannot be maintained for any length of 

 time, but that with time this maximum rate shifts to a lower temperature. 

 This is apparently due to the fact that there are two opposed reactions 

 involved. This has been shown graphically in Figure 14. If the point M 

 is taken as the optimum, this point is not a fixed one. Its position will be 

 determined by all factors which affect the rates of the opposing reactions 

 O A and D B, and these factors are manifold. In the case of photo- 

 synthesis the curve of inactivation, D B, is a function not only of tem- 

 perature but of other factors as well, such as light intensity and the 

 accumulation of the products of photosynthesis. Similarly the curve of 

 acceleration, O A, is a function not only of temperature but also of chloro- 

 phyll-content and certain internal factors. As a consequence the position 

 of the optimum point, M, is dependent upon the previous treatment of the 

 plant material in regard to temperature and illumination intensity and 

 the rate at which the temperature is raised to this optimum point. 



Blackman and Matthaei have shown that for cherry laurel the rate of 

 photosynthesis remains fairly constant at temperatures below 25°. Above 

 this temperature the initial rate cannot be maintained, but decreases with 

 time. It is highly probable that different species of plants differ as to the 

 point where this time' factor first becomes apparent. 



As has been stated, the inactivation of photosynthetic activity can be 

 brought about not only by higher temperatures but also by long exposure 



»-Kny, Ber. hot. Ges., 15, 396 (1897). Klebs, Biol. Centralb., 7, 166 (1887). 

 "'Fromageot, Compt. rend., 177, 779 (1923). 



