

1905. ] On Vegetable Assimilation and Respiration. —s 411 
reading. When a knowledge of this is important, and the light is variable, 
we have taken the temperature as often as every 3 minutes, all through the 
experiment. — 
The difference between the observed temperature of the water-bath and the 
determined temperature of the leaf gives a rough practical measure of the 
intensity of the radiation falling on the leaf, and in some experiments this 
has been calculated for each reading, and used to elucidate the con- 
temporaneous assimilation-values. 
If the temperature of the bath and also that of the leaf be plotted 
graphically, as in figs. 4 and 5, we get in their difference a continuous record 
of the approximate light intensity. 
During all the later experiments, records of the air shade-temperatures and 
of the readings of a black-bulb-in-vacuum thermometer in the sun have been 
kept, but we have not thought it important to publish these. 
A single extract (Experiment VI, July 24, 1904) may be given here to show 
the relation of the whole set of temperatures, 












Shade Egat Vacuum Bath- Leaf- 
Time. Illumination. tempera en thermometer | tempera- | tempera- 
ture eta: in sun. ture ture. 
in sun, : 
A.M. 18) —. *C. ~C. ae 
[Ll 2 yell eal 0 ee ana at 23 °9 24: °5 36 °5 19 °7 24 °3 
11.30 | Gleams of sun ......... | 241 24.°8 37 °5 1OGy 22° 4 
EAA AIP BUM .osiccdceene css | 6.248 25 °5 42 °0 20 °2 Zhavaa se 
Hi53) | Brillant sum............ 25 °8 28 ‘0 54 °5 20 °6 | Pa Un 
P.M. 
12.2 ) DoH oe aaa eee 24 °7 24 °6 39 0 19 -7 21°5 
2.20 | Steady dull ............ 24:0 25 ‘1 37 5 19 ‘8 21°5 
1.10 a aE eres 23-8 24-0 29-5 19 °3 21°5 
HPQO Clearing. s...c5266- 4000: 24:°5 24. °7 36 ‘0 19°7 21°9 
1.31 | Sun coming out ...... 25 °4 26 °4 41 -O 20 ‘0 25 °4 
142 | Brghtsun .........+. 26 °1 27 °4 48 °5 20 ‘6 28 *4 
1.51 | Sun and thin cloud a 25 °7 26 °7 50 ‘0 20 °8 27 4: 


However obscured the sun may be, it is always found that the diffuse 
light of the sky, according to its brightness, heats up the leaf 1°, 2°, or even 
3° above the temperature of the bath.* This demonstrates the greater 
effectiveness, in absorption of radiation, of the coloured and semi-opaque leaf 
over the colourless and transparent glass and water. 
Such high temperatures in ordinary leaves in the sun are somewhat 
* It is only after darkening the water-bath with a black cloth that the temperature of 
the leaf falls as low as that of the bath. 
