I 



Vegetable Assimilation and Respiration. 315 



Figs. 4 and 5 show that this material does not exhibit the effect described 

 by Treboux. Neither acidification increases the bubble rate appreciably. 

 In fig. 5 (A) the early rate is lower than the later : this appeared possibly 

 due to gas-pressure variation. Such instances of variation of rate without 

 change of conditions were extremely rare : at such times the light did not 

 appear to the eye to be as perfectly steady as usual. 



The next step was to obtain Elodea from a natural water, and in April 

 experiments were made with material from a chalky stream (at "Nine 

 Wells "). Here the result is quite different, as figs. 6 and 7 show. 



In fig. 6 the addition of acid about doubles the rate of bubbling in B. On 

 setting up the plant in the unacidified CO2 solution, it returns in D to its 

 original rate, to be again accelerated by acid. At C it is seen that stirring 

 causes a new transient " initial CO2 diffusion effect," with return to the 

 previous rate. If any of the increased bubble rate in B had been due to 

 making good a previous lack of oxygen in the solution, stirring would have 

 brought on a sudden drop, followed by a rising series of bubble rates, instead 

 of the sudden rise with a falling series, characteristic of the CO2 diffusion effect. 



In fig. 7 we have at first a repetition of the effect of fig. 6. The final 

 part provides a demonstration that the effect of added HCl is really due to 

 liberated CO2, on applying the theory of limiting factors. If enough CO2 is 

 present m solution, then the rate of bubbling will be controlled by the light 

 only as a limiting factor, and in such case, by definition, a further addition of 

 CO2 should not increase the rate of assimilation. If, now, all that HCl does 

 is to generate more CO2 locally at the surface of the plant, then in such a 

 case, with CO2 already in excess, addition of HCl should not increase the 

 bubble rate. This is what is found at E. It should be noted that the 

 " initial CO2 diffusion effect " is also absent in D and E, and is very small, 

 partly controlled by the light limit, at the beginning of B. We get thus 

 another line of proof of the nature of the acidification effect. 



We may safely conclude that Elodea, in chalky streams, does become 

 impregnated at its surface with CaCOs, though not enough to be visibly 

 affected, and that, after growing for some time in soft water, the shoots lose 

 this calcification. The Elodea grown in a tub owes its inability to give an 

 "acid effect" to this cause, and it was further proved that, when the 

 material from the chalky stream was placed, in April, to grow in the tub, it 

 ceased after two or three weeks to give this effect any longer (see Experiment 

 table on p. 318). 



Some further illustrations of the presence of the acid effect with fresh 

 material when CO2 is limiting and its absence when light is limiting are 

 afforded by the next two experiments. 



