Vegetable Assimilation and Respiration. 



317 



In fig. 8 the CO2 supply is well in excess of the light intensity, and no 

 increased bubbling results on addition of HCl. In fig. 9 we have a more 

 elaborate experiment, showing in its first part, sections A-B-C, a large 

 effect of acid when weaker CO2 is used, and in its second part, D-E, a small 

 effect when stronger CO2 is supplied; a third part, F, was carried out to 

 demonstrate that it was really the limiting factor of light which kept the 

 effect so small in E, for on simply bringing the light nearer, in F, the rate of 

 bubbling rose at once. 



In the next experiment of this series four separate shoots were set up to 

 bubble in one large jar containing 500 c.c. of CO2 solution ; the results of 

 acidification are tabulated below. Plant C came fresh from the chalk 

 stream, A had grown in the water-tub all the winter, while B and B' were 

 plants from the same habitat as C, but brought in some weeks earlier and 

 transferred to the tub. The bubble rates of the individual plants in the one 

 solution differed a good deal, ' partly determined by the sizes of the shoots 

 and partly by the different sized nozzles of the four bubblers (C was a very 

 short stout shoot, and its bubbler had a very large opening). To make them 

 easily comparable, the actual rates have been multiplied by a factor, given at 

 the head of the Table, to bring them all to the value of 10 for the original 

 rate in CO2 solution. On adding HCl to bring the acidity to 0"0008 IST, the 



changed ; D, solution poured off and a second half of original 3"33 per cent. CO3 



solution put in ; E, again HCl to 0'0005 N. 

 Fig. 7. — Elodea from chalky stream, two plants side by side in the same chamber — 



upper and lower records. Light = 12'3 ; temperature 18° C. A, in CO2 solution ; 



B, added HCl to O'OOlO N ; C, second half of solution used in A ; D, some strong 



stock CO2 solution added to make the CO2 very strong and superlimiting, in order 



to determine the light-limit value ; E, HCl added to D to O'OOlO N ; the light is 



now limiting and the effect of acid quite absent (see text). 

 Fig. 8. — Elodea from chalky stream in moderate light and strong COj, so that light is 



limiting. Light = 6 ; temperature 18° C. A, 3"7 per cent. CO2 solution ; B, HCl 



to 0-0005 N. 



EiG. 9. — Elodea from chalky stream : light intensity 12"3 in series A-E, in F 16. 

 A, 0'65 per cent. CO3 solution ; B, HCl to O'OOl N ; C, solution replaced by second 

 half of the original 0"65 per cent. COg ; D, 1'2 per cent. CO2 solution ; E, HCl to 

 O'OOl N : here the light limit is reached before any large acid effect is manifest : 

 the horizontal line at 12'5 b.p.m. marks the limiting light value ; F, the light 

 intensity is increased to 16 without alteiing the solution, and the bubble rate 

 at once goes up to 15 b.p.m. 



Fig. 10. — Elodea from chalky stream : Light = 12. A, in COj solution ; then in the five 

 following stages additions of HCl were made to bring the acidity up to the following 

 values ; B, 0-00013 N ; C, 0-00027 N ; D, 0-00054 N ; E, 0-0008 N ; F, 0-00107 N. 

 In G, the solution was changed for the second half of the solution used in A ; the 

 activity is not greatly depressed below the original value in A, In H, a greatly 

 increased concentration of CO2 was obtained by adding stock CO2 solution, and the 

 bubble rate goes up enormously. 



