114 CHAP. XV. C0 2 -SUPPLY AND PHOTOSYNTHESIS 



Bubbler-attachment. For long-continued experiments it 

 is therefore advisable to employ solutions containing about 

 8 mg. per ioo c.c. 



The Special Plant-Vessel 



For obtaining the characteristic curve under variation of 

 C0 2 , it is necessary that an identical plant should be used 

 for the complete investigation, since the curve is liable to 

 modification by the physiological condition of the specimen. 

 Special care has to be taken that the fresh introduction 

 of solution of a different strength does not cause any 

 mechanical disturbance, for a mechanical shock cannot 

 but affect the normal rate. The apparatus devised for 

 the introduction of different solutions without causing 

 mechanical shock to the plant has already been described 

 (see fig. 14) ; the different solutions are introduced by the 

 appropriate manipulation of the stop-cocks. 



The constant light employed was that of Pointolite at 

 about 1200 lux. Each investigation was completed in less 

 than an hour. 



The Photosynthetic Curve under Variation of 

 C0 2 -Concentration 



For descriptive purposes I reproduce (fig. 33) a charac- 

 teristic curve in which the ordinate represents the activity 

 and the abscissa the C0 2 -concentration of the solution in 

 mg. per 100 c.c. This curve may be regarded as typical, 

 the curves given by a dozen different specimens being 

 practically the same. 



Referring to the general curve, we find that it is straight 

 from a concentration of 3 to 7*5 mg., and that a turning- 

 point occurs after 8 mg. After this, photosynthesis tends 

 to reach a limit, and this limiting curve is not abruptly hori- 

 zontal. The above description holds good in the generality 

 of cases ; but in a few instances a depression was produced 



