7() 



be Ihe case. A Nvel sunimer is as a rule followed by an early 

 aiilumn (IVom the point of view of the forest vegetation). On the 

 other hand the production of CO2 is more abundant during a wet 

 summer. Compensation for the smaller amount of light is afforded 

 in thick and dark \voods by the there prevailing maximum CO., 

 concentration in the air. 



To make an estimate of the daily production of carbohydrates 

 in the shade-plants proves verj' difficult, on account of the strong 

 variations in the lighl-intensity and avaihible supply of carbon 

 (iioxide. A calculation can only be based upon continuous obser- 

 vations of the light and the CO<> concentration in the habitats of 

 the plants, and these observations I have not yet been able to 

 make. I shall therefore at present limit myself to an approximate 

 estimate of the necessary minimum conditions. 



The eslimations given in the present paper concern the relative 

 intensity of assimilation, which is somewhat lower than the actual 

 intensity, on account of the continuous respiration. Ecologically 

 however it is just this relative intensity that is of interest. The 

 point where respiration and assimilation counterbalance each other 

 is the point where the curve cuts the abscissa. With normal CO2 

 percentage this point of equilibrium lies at light-intensity j^^ to 

 T4ö^' With higher light-intensity the assimilation gives a positive 

 result, with lower light-intensity the respiration dominates and the 

 leafs funds of carl)ohydrates are diminished. Owing to the higher 

 CO2 tension prevailing in the forest, the point of equilibrium should 

 probably be pushed back to a light-intensity of about -^\-q. — This 

 relatively low intensity however does not denote the actual minimum 

 of existence. For in the nocturnal respiration carbon dioxide is löst. 



The shade-plants examined have the foUowing intensity of respi- 

 ration, calculated per 50 cm.-, 18^, and 1 hour. 

 Oxalis Acetosella O.30 mg. 



Melandrium rubrum 0,20 » 

 Stellaria nemorum O.si» » 



^ The value obtained by Boysen-Jensen (1918, p. 249) for the point of equilibrium 

 in Oxalis, viz. O.2 (Bunsen unit.s X 100), corresponding to about jj- aceording to 

 my soale, seems to me somewhat too low. The value is probably to be explained 

 by the fact that with Boysen-Jensen's method .some COg is always lo.st in the 

 absorption, so that e. g. the respiration values must tum ont too low. A per. 

 fectly exact calculation of the position of the point of equilibrium is naturally I 

 rendered more difficult by the heterogeneity of the material, whatever be the 

 method employed. 



