1905. | On Vegetable Assimilation and Respiration. 443 
Let us now compare the results with these two very different types of 
leaf :— 
Cherry-laurel... 0°28 sunlight gives 0°0116. .. fullsun = 0:0414 
Helianthus ...... 0°62 sunlight gives 00224. .. full sun = 0°0361 
The sun with the cherry-laurel was 14 hours before noon, and with 
Helianthus was 13 hours after noon, so no correction is to be applied on that 
account.* 
The larger value was obtained on August 16, and the smaller one on 
August 8, when the brightness of the sun at noon is about 3 per cent. greater, 
so that correction for the greater air mass traversed by the rays of the 
lower sun of the later date would tend to bring the numbers further apart by 
this amount. 
Apart from any correction there is a fair agreement between these values 
as indicating that the full sun, about noon, about the middle of August, 
radiates enough light on to 50 sq. cm. of leaf surface, normally exposed, to 
reduce 0:04 gramme COz per hour. At the summer solstice the value should 
be about 6 per cent. larger. 
There is, however, a biological correction, legitimately to be applied to the 
two values, which brings them much closer together. This turns on the fact 
that the value for cherry-laurel was determined 14 hours after starting 
assimilation, while for Helianthus it was 5 hours after starting; the “time- 
factors” are different. 
There is evidence, to be considered in a separate paper, that the time-factor 
at high temperatures is not of the nature of a limiting factor, but involves a 
falling-off of the efficiency of the chloroplast machinery of such a nature that 
it takes, to put it crudely, continually more energy to reduce a given amount. 
of COz The chloroplast will, at any time, require a fixed amount of light- 
energy, but will reduce less and less CO2; also, with any less amount of energy, 
there will be a proportional decrease in the amount of CO, reduced. If, then, 
at any one moment at the temperature of 29°5 with cherry-laurel, 0°36 light 
gives maximal assimilation, then, according to this interpretation, it would 
give it also at any other moment. | 
So, also, with Helianthus, 0°69 light will always give just the maximal 
assimilation at this same temperature. 
What, then, is the ratio of the maximal assimilation at about 29°5 C. for 
these two leaves? Fig. 6 shows that, for those times at which there is 
* Crova has shown that after noon the more abundant water vapour renders the 
atmosphere less transparent to heat radiations than before noon, but there is no evidence 
that this holds for light. 
