— 242 — 



Escombe and by me 1 ) agree very well, and the intensity of the C0 2 

 assimilation in quickly growing light plants can be estimated at 

 4 — 6 mg per 50 cm 2 per hour by 20°. The objection urged by Thoday 

 against the experiments of Brown and Escombe that the leaves in 

 their experiments not were fully turgid, can scarcely be maintained 

 against my experiments. On the other hand the experiments of 

 Sachs and Thoday aimed at estimating the greatest possible C0 2 

 assimilation ; these experiments are carried out in direct sunshine ; 

 the air temperature in the experiments of Thoday was 27 — 29° 

 and the leaf temperature will probably have been some degrees 

 higher. When we calculate the results of the experiments obtained 

 by the air current method at a temperature of 35° we will (if the 

 light or the G0 2 supply do not act as limiting factors at this tempera- 

 ture) get a value of about 18 mg C0 2 . Hence it appears, that the 

 difference in the values obtained with the two methods is not so 

 great as it might appear. But to consider the values obtained by 

 Sachs and Thoday as the mean for the GO 2 assimilation over a longer 

 period of time will not be correct. First the sun does not shine every 

 day, and it is only in direct sunshine that the high leaf tempera- 

 tures occurs, and for the second, in a protracted period of sunshine 

 the plants will quickly come to suffer from lack of water, the sto- 

 mata will be closed and the G0 2 assimilation will be diminished. 

 I have found that to be the case in the month of June in 1917 in 

 Denmark. Indeed, a G0 2 assimilation of 18 mg per 50 cm 2 would 

 mean a production of dry matter higher than has ever been found 

 in our climate. I shall return to that point below. 



From the assimilation curves given in the figs. 3 and 4, the 

 * curves for the real assimilation (the apparent assimilation + the 

 C0 2 given off by respiration) can easily be constructed by dis- 

 placing the curves parallel to the vertical line until they begin 

 at the zero point. 



2. Respiration. As will be seen from the curves the respiration 

 for the leaves of the four plants is the following (mg C0 2 per 50 cm 2 

 of the upper surf. + 50 cm 2 of the lower surface): 



Sinapis alba 0.8 



Senecio silvaticus 0.7 



Rumex acetosella 0.5 



Sambucus nigra (sun leaves) 0.3 



*) To be sure I have not worked with Helianthus; but I am inclined to 

 believe that the C0 2 assimilation for this plant will not be greater 



than for Sinapis. 



