% 
1905. ] On Vegetable Assimilation and Respiration. 429 
the leaf was directly determined thermo-electrically, and. an attempt was made 
to keep the leaf-temperature uniform by altering the temperature of the 
water circulating through the bath. When the leaf has to be exposed to 
direct sun then the gas-supply to the water-heater is diminished, or the rate 
of water circulation is increased, so as to compensate for the warming up 
produced by direct insolation. 
This was a bright day, on which the sky was more or less closely packed 
with brilliant white cumuli. 
For the first reading the wooden tube was fitted on to the bath and the tube 
pointed, not towards the sun but to a part of the sky away from the sun and 
nearer the zenith. A comparatively small number, 0:0053, was obtained (but 
not so small as expected), corresponding to the patch of diffuse hight that finds 
its way down the tube. The second reading is taken in full diffuse hight behind 
the shadow-board, and reaches the high value of 0°0212. This, as might well be 
expected, represents the full effect of the light, and is not limited by the 
temperature ; Curve A in fig. 2 shows that a very much larger assimilation 
would be possible at this temperature. Then several readings in direct 
sunlight only are taken by replacing the wooden tube and keeping it pointed 
to the sun. The assimilation of 0°0177 in the first of these readings is much 
smaller than the one in diffuse light, but subsequently a more nearly equal 
reading of 0°0195 (or, allowing for the three minutes of cloud, 0°0209) is 
obtained. This whole experiment shows how efficient total diffuse sky-light 
may be in relation to poor direct insolation alone. 
The weather broke up at 3.42, and so two estimations of the respiration in 
the dark were made, and the values here obtained at.29°5 C. are part of the 
data for the respiratory curve in fig. 2. | 
In Experiment X with Helianthus the leaf-temperature was, for most 
readings, kept much lower, and it will be seen that therefore the amounts of 
assimilation are limited by the temperature. Jain fell on and off throughout 
the day, and no sun appeared till the last reading at 3.42 p.m. The chamber 
faced south and 30° above the horizon throughout the day, unmoved, thus 
receiving a large amount of diffuse light from the sky. When the sun 
appeared for the last reading the shadow-board was used to intercept its 
rays. The whole experiment then is conducted in. diffuse light of varying 
brightness. | | 
For the first four readings the temperature was kept down to about 18° C., 
and, again, in the last two the temperature was the same. In all these 
readings except the first, which was low, due to the extremely overcast leaden 
sky, the assimilation numbers are remarkably uniform, 0:0089, 0:0090, 0:0089, 
0:0089, 0:0092; while the light varied up and down, being especially brighter in 
