CARBON DIOXIDE MOLECULES AND CARBONIC ACID IONS 



889 



Myriophyllum grew in a locality where the pH in summer was as high as 

 9-10 (corresponding to a ratio of 100 HCOs" to 1 CO2), while Fontinalis 

 was gathered in a locality where the water contained very Httle bicarbonate, 

 but as much as 30 X 10"^ mole per liter of free CO2 molecules (a remarkably 

 high figure, if one recalls that water equilibrated with the free atmosphere 

 contains only about 1 X 10 ~^ mole per liter CO2). 



The behavior of the two species is illustrated by figures 27.1 A and B. 

 One can either accept these figures as evidence of direct participation of 



E 

 E 



Q. 

 O 



c 



0) 



u 



0) 

 Q. 



o 



3 

 O 



o 



Q. 



05 



10 10 20 30 40 50 60 



CO, (total), in 10'' mole/l 



0.5 10 5 10 



CO2 (total), in 10"' mole/l. 



15 



Fig. 27.1. (A) Photosynthesis in Fontinalis antipyretica, at 15,000 lux, 22° C, as 

 function of total carbonate concentration; (B) Photosynthesis in Myriophyllum spicatum, 

 at 37,000 lux, 20° C, as function of total carbonate concentration (after Steeraann- 

 Nielsen 1946). 



HCOs" ions in the photosynthesis of Myriophyllutn (or, more exactly, of 

 comparatively easy penetration of HCOs" ions into the cells of this plant), 

 or one must assume that, for some reason yet unknown, the buffering action 

 of bicarbonate was much more effective with Myriophyllum than with 

 Fo7itinalis. 



It will be noted that the "free carbon dioxide" curves are practically identical for 

 both species; in other words, if we attribute the shapes of these curves to carbon dioxide 

 depletion, we must assume that the latter has been equally strong in both experiments. 

 We must then assume that addition of about 100 HCOs" to 1 CO^ has had very little 

 effect on (lei)letion in the case of Fontinalis, but had reduced it by about one third in 

 that of Mijrioplujllum. Whether this explanation is plausible is difficult to say without 

 the knowledge of various relevant factors, such as the absolute rate of photosynthesis 

 per unit area, shape.s of the two plants (ratio of volume to surface; cf. page 905) and 

 the efficiencj' of stirring. 



In chapter 13 (Vol. 1) we mentioned the effect of canons on the rate of photosynthesis, 

 in particular the difference between the rates in sodium carbonate and potassium carbon- 

 ate buffers, as observed by Pijson and later by Pratt. Some additional information on 

 the unfavorable effect of sodium bicarbonate was given in chapter 25. Steemann-Nielsen 



