84 



Carbon Assimilation. 



In order to examine the intake of carbon dioxide by submerged 

 water plants, a stream of water containing carbon dioxide dissolved 

 in it was passed over the leaf. By estimations of the carbon dioxide 

 that had passed over the leaf in a definite period the rate of intake 

 of the gas could be measured. The procedure employed with land 

 plants was somewhat modified. A description of the method is 

 given in the eighth paper in Blackman's series of researches 

 (Blackman and Smith, 1911 a). 



With small-leaved plants such as Fontinalis and Elodea, sprigs 

 of the plant were used instead of single leaves as in experiments 

 with Cherry Laurel or Helianthus. In most of the experiments 

 medium temperatures and medium illumination were employed. 

 The value taken for the carbon dioxide supply is tiie mean value of 

 the carbon dioxide concentrations of the liquid before and after 

 passage through the chamber. The ' real ' assimilation was 

 calculated as in the experiments with land plants already described. 



The results obtained with both Fontinalis and Elodea are 

 shown in the accompanyinjj curves (Pig. 13). In the weaker 



o" 

 u 



o 



2 'Oi6 



■0O8 



•Ol -OX -03 0^ -05 '06 



Carbon dioxide supply in grams of COj per 100 c.c. water. 



Fig. 13. Curves illustrating the influence of the magnitude of carbon 

 dioxide supply on assimilation. E, Elodea; F, Fontinalis {after Blackman and 

 Smith). 



solutions of carbon dioxide, the assimilation increases directly 

 with increase of carbon dioxide supply. Neither light nor temper- 

 ature are limiting. In each case, however, a point is reached where 

 increase in carbon dioxide supply is no longer accompanied by a 

 corresponding increase in assimilation. The latter remains constant 

 however much the carbon dioxide supply is increased. Here, 

 either temperature or illumination is limiting the rate of assimilation. 



