158 Ingvar Jorgensen and Walter Stiles. 
Besides a large number of similar determinations by Blackman 
and Matthaei made at various temperatures with light the limiting 
factor when natural illumination was employed a few confirmatory 
measurements were made with the water plant Elodea by Blackman 
and Smith (1911 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 temperature becomes a limiting 
factor. The earlier experiments of Reinke (1883) also support the 
conclusion. 
From the results of his experiments Blackman concludes that 
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 Ceratophyllum and 
Potamogeton. 
