1905. | On Vegetable Assimilation and Respiration. 453 
temperature which a Helianthus leaf actually reaches in the sun in the open 
air is probably lower than with cherry-laurel by virtue of greater trans- 
piration. 
While then cherry-laurel in the sun would certainly have its assimilation 
limited by its temperature, Helianthus might not be thus limited when it 
attained a similar temperature. If, as seems probable, it only attains much 
lower temperatures, it also might be limited in sun alone, and certainly so 
when illuminated by direct insolation plus bright diffuse light. 
Besides the illumination and the temperature there is a third factor which 
may function as a limiting factor. This is the partial pressure of COs in the 
atmosphere surrounding the leaf. In all our experiments the air-current has 
been enriched with CO, so that only in one case, and that unintentionally, 
has the CO2-supply been a limiting factor. How different is it in Nature! 
All the considerations that we have adduced regarding the extreme values 
of assimilation with full intensity of illumination and the highest temperatures 
come to nought, at least, as far as their direct existential import, when we 
realise that nowhere in nature is there sufficient carbon-dioxide in the 
environment to permit of anything approaching such intensity of assimilation. 
We may take it that the COz in the atmosphere rarely exceeds three parts 
in 10,000 (except indeed in London fogs), and we have to enquire to what 
maximum of assimilation this will limit a leaf in the open air. We have 
made no experiments on this point ourselves, but the experiments of Sachs 
and of Horace Brown furnish us with data. 
Horace Brown* has shown that a surface of caustic alkali exposed in the 
open air containing 3 parts in 10,000 of COs, absorbs per hour and per square 
metre 1200 c.c. of this gas when the air is still and up to 1500 cc. if the air 
is agitated by wind. | 
Brown and Morris} have also determined the amount of CO: taken in by a 
leaf of Helianthus in the open air, by measuring the increase of dry weight 
after the method of Sachs. 
On a bright warm day a cut leaf gained 1 gramme dry weight per hour 
per square metre and on a dull day a cut leaf gained 0°985 gramme, this gain 
is to be treated as if it were all carbohydrate of the cane-sugar type. 
Expressing the results in grammes CO2 per 50 sq. cm. of area per hour, we 
get the following absorptions :— 
* Address to the Chemical Secticn, British Association, 1899. 
t+ ‘Journal of the Chemical Society,’ 1893, p. 604. 
