
$ 
1905.] On the Physiological Processes of Green Leaves. 71 
the walls of which are non-reflective and are maintained along with the enclosed 
air, at a perfectly uniform temperature which we will denote by 6. We will 
further assume that at the commencement of the experiment the leaf is also 
at this same temperature @. | 
If the air of the enclosure is saturated with water-vapour, and there are no 
chemical changes going on in the leaf attended with evolution or absorption 
of energy, there will be complete thermal equilibrium in the system, all parts 
of which will remain at temperature 6.* 
Since, however, active respiration is going on in the leaf-cells, this static 
thermal condition cannot be exactly maintained, and the leaf-lamina will 
consequently tend to rise slightly in temperature. 
How small is the thermal disturbance due to this cause will be apparen 
from the following considerations. 
The amount of carbon dioxide evolved is a measure of the oxidation of the 
leaf-substance used up in the respiratory process, hence, provided we know 
the nature of the oxidized substance, and its heat of combustion in absolute 
units, we can determine the lhberated energy corresponding to the appearance 
of a given amount of carbon dioxide per unit-area of leaf per unit-time. 
We may safely assume that the substance which is used up in respiration 
belongs to the class of carbohydrates. Direct evidence of this has been 
given by Brown and Morris,} who found that the disappearance of the sugars 
and starch of leaves of Tropceolum placed in the dark corresponded very 
closely with the loss of weight due to respiration.t 
The heat of combustion of the different carbohydrates for equal weights of 
contained carbon varies only within narrow lmits, and we may therefore, 
without serious error, safely refer the carbon dioxide of leaf-respiration to a 
sugar of the hexose class, having a heat of combustion of about 3760 calories 
per gramme. 
The actual amount of carbon dioxide respired by a leaf varies considerably 
* It is here assumed that the partial pressure of the water-vapour in the interspaces of the 
leaf is in equilibrium with the vapour-pressure of a plane surface of water at the same tempera- 
ture. Should it ever be proved that transpiration into a saturated atmosphere is really possible 
when the whole system is in perfect thermal equilibrium, the cause must be sought either in the 
increased vapour-pressure of minute convex surfaces, such as give rise to the distillation of small 
drops into large ones, or, in some such properties of colloids, as those suggested by the 
observations of J. M. van Bemmelen and y. Schréder, which indicate that a higher tension 
of water-vapour can be maintained over a “gel” than corresponds to a plane surface of 
water at the same temperature. 
+ ‘Journ. Chem. Soc. Trans.,’ 1893, p. 671. 
{ Compare also the results of T. C. Day (‘ Journ. Chem. Soe. Trans.,’ 1880, p. 658), who showed 
that the ratio of the carbon dioxide and water liberated during the germination of barley 
corresponds closely with that required for the combustion of a glucose. 
