Carbon Assimilation. 39 
Table XLV. 
Percentage of Radiant Energy Incident on the Leaf used in 
Assimilation (Data from Puriewitsch). 
Species. 
Date. 
Dii ration 
of Experi¬ 
ment in 
hrs. & min. 
Total Inci¬ 
dent Energy 
per sq. cm. 
in gm.-cal. 
Increase in 
heat of com¬ 
bustion pers. 
cm. in g.-cal. 
Intensity of 
Radiant Energy 
in gm.-cal. per 
sq. cm. per min. 
Percentage 
ofsun energy 
used in 
Assimilation. 
Acer platanoides 
30 May, 1912 
6.0 
361-03 
2-208 
1-003 
0-6 
1 * 11 
2 June, 
11 
5.0 
162-59 
1-332 
0-542 
0-81 
* 1 » » 
13 ,, 
11 
6.0 
240-33 
6-508 
0-667 
2-7 
1 1 11 
19 ,, 
1 1 
5.0 
202-20 
2-630* 
0-674 
1-3 
Helianthus annuus 
11 „ 
11 
4.30 
132-48 
5-977 
0-454 
4-5 
Polygonum sacchalinense 
31 May, 
1 1 
1.20 
70-85 
5509 
0-885 
7-7 
11 11 
3 June, 
11 
3.0 
122-33 
5-076 
0-679 
41 
11 11 
16 „ 
1 1 
1.50 
97-62 
2-585* 
0-887 
2-6 
11 11 
17 „ 
11 
2.20 
12318 
4-656 
0-880 
3-7 
11 11 
21 „ 
1 1 
5.0 
136-81 
1-540 
0-456 
11 
11 . 11 
23 „ 
11 
5.0 
177-00 
4-514* 
0-590 
2-5 
Saxifraga cordifolia 
6 „ 
1 1 
2.20 
68-16 
3-450 
0-487 
50 
*The values marked with an asterisk were actually observed. The 
remaining values in this column were obtained by calculation. 
have been possible to correct. 
We may now attempt to correlate the results of Brown and 
Escombe and of Puriewitsch with those of Blackman on light as a 
factor in assimilation. It will be recalled that on Blackman’s view of 
limiting factors, if we commence with a very low light intensity, 
increase in light (radiant energy) will result in a proportionate 
increase in assimilation until some other factor, such as carbon 
dioxide supply, is limiting the rate. The curve connecting the light 
intensity and the rate of assimilation will be of the form already 
shown in Fig. 5. As regards the proportion of the radiant energy 
used in assimilation, this should remain constant on Blackman’s 
view so long as light is the limiting factor, for the rate of assimilation, 
and consequently the energy used for it, is directly proportional to 
the intensity of the light. But when the light is increased so that 
some other factor is limiting the rate of assimilation, then if that 
factor remains constant, increase in light intensity will result in a 
decrease in the percentage of radiant energy used in assimilation. 
In many of Brown and Escombe’s experiments the intensity of 
illumination is roughly inversely proportional to the proportion of 
the energy used in assimilation, but this relation is not by any means 
