— 240 — 



substances or climatic factors on the production of matter. Mitscher- 

 lich formulates the minimum law of Liebig as follows : The amount 

 of the crop rises on increasing the supply of a nutritive substance 

 acting as limiting factor but the increase it not proportional to the 

 supply but decreases gradually when approaching the point where 

 the factor in question is no more limiting. 



The curves are characterised by 3 points: 1. The value for the 

 light intensity 0 (= the respiration of the leaves), 2. The intensity 

 of light, in which respiration and C0 2 assimilation are in equilibrium, 

 and 3. The maximum value for the C0 2 assimilation. 



Considering the assimilation curves it is obvious that the dif- 

 ference between them is small. Only the curve for the sun leaves 

 of Sambucus has lower values than the other 3. The values for the 

 3 cardinal points is as follows: 









The maximum 





Respiration, mg 



Intensity of light, 



value of the C0 2 





per hour per 50 cm 3 



in which respira- 



assimilation per 





of the upper surf. 



tion and G0 2 as- 



50 cm 2 of the up- 





-f 50 cm 3 of the 



similation are in 



per surface -f 





lower surface 



equilibrium 



50 cm 2 of the 

 lower surf. 





0.8 



1.0 



6.0 



Senecio silvatic. . . 



0.7 



0.6 



? 



Rumex acetos 



0.5 



0.5 



4.5 



Sambuc. nigra (sun 









leaves) 



0.3 



0.7 



2.3 



It is now of interest to compare the maximum value with that 

 obtained by other investigators (cf. the table pag. 241). 



It may be seen that the difference between the estimations 

 of the different authors is considerable. In reality the estimations 

 can without difficulty be divided into two groups: those that are 

 carried out by the half leaf method of Sachs and those that are 

 carried out with the air current method of Kreussler. With the 

 first method the value of the C0 2 assimilation is estimated to about 

 15 mg CO g per 50 cm 2 per hour, with the latter to 4 — 6mg. As 

 might be expected the values obtained in my experiments lies 

 nearest to those obtained by Giltay and Brown and Escombe. 



As pointed out by Thoday the reason for these discrepancies 

 may be sought for in the difference in the surrounding conditions, 

 especially the temperature. The values obtained by Brown and 



