

1905. | On the Thermal Hmassivity of a Green Leaf. 129 
No. 2 Coil_—Leaves arranged with stomatiferous surfaces inwards. 
Duration of experiment 2h. 9m. = 129 minutes. 
Area of leaf surface exposed = 139°4 sq. cm. 
Water lost by transpiration during experiment :— 
INGE: S22 Sac 2. ... 0°640 gramme 
pe ee ee ORESO Le 
Difference ...... QC; 4 
The mean temperature difference betwen the two pairs of leaves as indicated 
by the integration of the drum record was 7:05 scale divisions = 1°-41 C. 
Since the difference in the amount of water vaporised by the two pairs of 
leaves is 0510 gramme and the latent heat of vaporisation of water at 18°°6 C. 
is 593°6 water-gramme-units (calories), it follows that 0°510 x 593°6 = 302°7, 
represents in calories the excess of energy which must have entered the 
cooler pair of leaves from their surroundings during the experiment, an 
excess, which is conditioned solely by the temperature gradient of 1°41 
represented by the temperature difference between the two sets of leaves, for 
all other conditions are similar. 
Since the surface area of the exposed leaves in each case is 139-4 sq. cm., 
and the time occupied by the experiment is 129 minutes, the thermal emissivity 
of this particular kind of leaf expressed in calories per square centimetre of 
surface per minute, per 1° C. temperature excess, is represented by 
302°7 
EE ES 2 ] 1 
139 x 1394x141 0:01194 calorie, 
or 0:000199 calorie per second.* 
* We have assumed that the thermal emissivity of the upper and lower surface of the leaf- 
lamina is identical. 
Strictly speaking it is the mean thermal emissivity of the upper and lower surfaces which is. 
measured by this method, but these mean results can be applied to the determinations of the 
emissivity of the entire leaf, including both sides, just as well as if we knew the respective 
_ emissivities of the two surfaces separately, since it is immaterial whether we take the-total 
emissivity of the leaf as twice the mean value for the two sides, or as the sum of the separate 
emissivity-values, if these are known. 
It is highly probable that even in glabrous leaves the emissivities of the upper and lower 
surfaces are not quite identical, for ‘‘ surface emissivity” must depend to some extent on the heat- 
conductivity of the underlying tissue, even in a very thin lamina such as that of an ordinary 
foliage leaf. One would consequently expect that heat would flow into and out of the leaf some- 
what more readily through the cuticle and epidermis overlying the more closely-packed palisade- 
parenchyma of the dorsal side, than through those portions overlying the spongy parenchyma of 
the ventral side with its numerous air-spaces. 
We propose at some future time to investigate this question, which can be solved in the 
following manner: Having determined the thermal emissivity of two pairs of leaves, one of 
VOL. LXXVI.—B. K 
