122 
Ingvar Jorgensen and Walter Stiles. 
Table V. 
Assimilation by the two Surfaces of various Leaves 
illuminated on Upper Surface. 
CO, assimil- 
Ratio of CO 2 
Ratio of stom- 
Plant. 
Time in 
Leaf area 
ated in c.cs. 
assimilated 
atic distribution 
hours. 
in sq. cms. 
Upper 
Upper 
Upper 
Lower 
Lower 
Lower 
59-44 
4-34 
100 
100 
Colchicum speciosum 
5-75 
3-26 
72 
119 
28-27 
3-90 
100 
100 
Senecio macrophyllus 
4-75 
3-60 
92 
126 
4-25 
5-80 
100 
100 
»» > > 
28-27 
4-20 
72 
126 
50 
59-44 
5-70 
100 
100 
Rumex alpinus 
8-90 
144 
269 
59-44 
7-50 
100 
100 
5-5 
._ 
> > » y 
9-81 
130 
269 
2-0 
76-97 
2-20 
100 
100 
Nuphar advenum 
000 
0 
0 
000 
0 
0 
Catalpa bignonioides 
1-85 
79 03 
4-91 
100 
100 
79-03 
0 00 
0 
0 
2-3 
_ _ 
>> »» 
8-96 
100 
100 
Brown and Escotnbe’s results confirm Blackman’s in regard to 
leaves with the stomata confined to one surface. With leaves 
hearing stomata on both surfaces, when illuminated on the upper 
surface there is always less intake of carbon dioxide by the lower 
surface than might be expected from the relative distribution of the 
stomata over the two surfaces, whereas the ratio of carbon dioxide 
respired from the two surfaces follows very closely the ratio of 
stomatal distribution. 
Brown and Escombe explain this result in the following way. 
During respiration, if there is a steady evolution of carbon dioxide, 
the rate at which this will escape from the leaf will be independent 
of the degree of opening of the stomata, for should the stomatal 
aperture decrease, the partial pressure of carbon dioxide inside 
the leaf will correspondingly increase and the rise in ‘ diffusion 
potential ’ will counterbalance the effect of diminished stomatal 
aperture. 
In the case of assimilation, on the other hand, the ‘ diffusion 
potential ’ will remain constant, for the partial pressure of the 
carbon dioxide diffusing inwards varies constantly from 0-0003 
atmosphere outside the leaf to zero where there is complete 
absorption of the carbon dioxide. Hence, if the stomatal opening 
