Carbon Dioxide Supply. 83 



Besides a large number of similar determinations by Blackman 

 and Matthaei made at various temperatures witii ligiit the limiting 

 factor when natural illumination was employed, a few confirmatory 

 measurements were made with the water plant Elodea by BlacUman 

 and Smith (191 1 b) ; the results have already been shown /graphically 

 in this chapter (see Fig. 6). These writers have also shown that 

 Pantanelli's results (1903) rightly interpreted, indicate that assimi- 

 lation is directly proportional to the intensity of light used until 

 either carbon dioxide supply or temperatui'e becomes a limiting 

 factor. The earlier experiments of Reinke (1883) also support the 

 conclusion. 



From the results of his experiments Blackman concludes thatj 

 where temperature and carbon dioxide supply are in excess the rate] 

 of assimilation is proportional to the intensity of illumination. 

 There is thus for every temperature a minimum value of the light 

 intensity which is sufficient to allow the maximum assimilation rate 

 to take place at that temperature always presuming no other factor 

 is limiting. By using perforated screens in front of the leaf to cut 

 off part of the sunlight, Blackman and Matthaei were able to show 

 what proportion of sunlight was required to give the maximum 

 possible assimilation at 29-5"C. Thus it was shown that in bright 

 sunlight during the middle of the day in August the maximum 

 assimilation possible at 29'5°C in the case of Cherry Laurel was 

 given by 0'36 of full sunlight and in the case of Helianthus by 0*69 

 of full sunlight. It would be expected that Helianthus was therefore 

 capable of a much higher rate of assimilation than Cherry Laurel 

 at the same temperature, and this is indeed the case (cf. Fig. 11). 



Indeed Blackman and Matthaei show that when light is the 

 limiting factor equal areas of different plants equally illuminated 

 produce the same amount of assimilation. Blackman and Smith 

 (1911 b) have shown that the same law holds with water plants 



(cf. Fig. 13). 



With the bearing of these results on the general question of 

 energy in regard to assimilation we propose to deal later. 

 D. Carbon Dioxide Supply. 



The influence of carbon dioxide supply upon assimilation has 

 been investigated by Blackman and Smith in the case of submerged 

 water plants. Elodea was chiefly employed for this purpose, but 

 experiments were also made with a water-moss Fontinalis, and a few 

 isolated observations were also made with C^ratophyllutn and 

 Potamogeton. 



