452 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
persist later and that assimilation actually overbalances respiration until 
sunset, and continues to cause a gain of material to the leaf up to that time. 
From natural illumination in relation to assimilation-intensity we may 
turn now to the question of natural temperature in the open air. 
The temperature of a leaf in the shade will equal the air-temperature 
within a degree or so, and the temperature may therefore well be so low that 
it would prevent the light producing its full effect, and the temperature would 
then in Nature, as in many of our experiments, be a limiting factor. 
If we assume that a cherry-laurel leaf in the shade is just about the air- 
temperature, our curve of assimilition-maxima in fig. 2 shows us at once to 
what extent the assimilation would be limited by the temperature. Let us 
take the particular case of August 7, when the diffuse light was sufficiently 
bright to give an assimilation of 0°0192 in the shade. We construct the 
following table of the effect at different temperatures :— 
Percentage 
Leaf- Assimilation of available light 
temperature. possible. utilised. 
LSC. 0:0075 39 
22 0:0097 50 
28 00135 | 70 
In direct sunshine in open air the case would generally be different, and 
the absorption of total radiation by the leaf would so raise its temperature 
as to make it capable of utilising a larger part of the rays specific to 
photosynthesis. 
We may consider further cases from our work. The cherry-laurel leaf in 
brilliant sunshine in the open air reaches possibly a temperature of 
9° to 13° C. above the mercury thermometer in the sun, say a temperature of 
39° C. fora bright hot day ; this corresponds theoretically to an initial assimi- 
lation value of 0°0352 gramme COz per 50 sq. cm. per hour, which is about 
three-quarters of 0°047 gramme COs, the calculated “initial” value for full 
intensity of August sunshine, and distinctly less than the possible value of 
full sunshine plus general diffuse light at the summer solstice. 
For Helianthus the temperature-coefficient is larger and the temperature 
maxima run up very rapidly at high temperatures. We have as yet no data 
for this leaf above 30° C., but continuing the curve upwards freely the 
theoretical initial value of 0°05 might even be attained at 38°C. It seems 
clear that this leaf might for a short time utilise the whole of the specific 
radiation in full August insolation at some temperature above 35°C. The 
