
1905. | On Vegetable Assimilation and Respiration. 445 
1:1:°95. We have, however, proved in the last section that at this higher 
temperature Helianthus requires twice as much light to reach its maximum 
as does cherry-laurel. 
This suggests that the same amount of COs is reduced in both plants by 
the same intensity of light. At 18° C. Helianthus would then require only 
1-2 times as much light as cherry-laurel for its maximal activity. The 
limitations set upon the activity of leaves by various temperatures would 
then be secondary in nature, and be merely superposed upon a primary and 
uniform relation between energy absorbed and work done. 
The only observations that militate against this uniform relation are 
those dealing with the decline of assimilatory activity with continued 
exposure to high temperatures. These effects of the “time-factor” appear 
not to be such as a limiting factor would produce, but rather to indicate a 
smaller economic coefficient of activity. 
This evidence must be postponed for the present, but we propose to give 
now experiments to prove that at temperatures not involving the time- 
factor, equal intensities of light produce the same amount of reduction of 
CO, even with very diversified types of leaf. 
These experiments were carried out with artificial light (Keith high- 
pressure incandescent gas light) just in the same way as the experiments in 
“ Assim. and Resp. III.” 
The leaf-chamber was contained in a water-bath at a uniform temperature 
of 25°C. The source of light was removed to such a distance that it only 
caused an assimilation of about 0:0085 gramme CO, per hour per 50 sq. cm. 
leaf-area. It was thus well below the assimilation-maximum for the 
temperature, which is, with cherry-laurel, 00114. This, of course, is of 
fundamental importance, and ensures that the amount of assimilation will 
really be a measure of the light-intensity. 
The light used has an arbitrary intensity depending on its distance from 
the bath; its intensity was not determined in any optical unit, but is 
expressed in terms of its reducing power for CO2 with a cherry-laurel leaf. 
We first compared a leaf of Tropeolwm with one of cherry-laurel. The 
experiments were as follows :— 
