444 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
evidence as to both, the value for Helianthus happens to be 1°95 times that 
for cherry-laurel. We should expect Helianthus to require 0°71 sun if cherry- 
laurel requires 0°36, and what it does require is 0°69 of sunlight 8 days 
nearer the summer solstice, when the sun is 3 per ceut. brighter. This 
correction for the time-factor then brings the estimations into extraordinary 
close harmony, some of which is no doubt accidental, and may vanish with 
further experiments. 
If the hypothetical initial values of the temperature maxima at 29° C. 
really hold for any appreciable time, and the time-factor has the significance 
here attributed to it, then the full sunlight will have a higher absolute 
photosynthetic value than the observed value. For, initially :— 
0:36 sun, on August 16, would give...... 0:0167 
and 0°69 sun, on August 8, would give...... 0°0325 
if these are the initial maxima for cherry-laurel and Helianthus respectively 
at this temperature. 
On this basis— 
Full sun, on August 16 = 0°0464 
Full sun, on August 8 = 0:0471 
which values have about the right proportion for the brightness of the noon 
sun at these two dates. 
The photosynthetic value of the full noontide sun at the summer solstice 
would then be about 0°050 gramme CO: per 50 sq. cm. of leaf surface 
normally exposed at this latitude. | 
These refinements of correction have no great significance till more 
estimations have been made, but somewhere between 0°05 and the 
uncorrected value of 0:04 the actual value will lie. 
Section VI.— The Specific Assimilational Characteristics of Diverse Leaves. 
This section will be devoted to some physiological considerations suggested 
by the differences in functional activity of the two types of leaf with which 
we have worked. | 
For one thing we find that the two leaves do not have the same assimilation- 
maxima at identical temperatures. This in itself does not greatly surprise, 
and one might expect that the “less active” leaf of cherry-laurel would 
perhaps reduce less CO, for the same amount of energy—that there would 
be different specific economic coefficients of chloroplast activity for the two 
leaves. In that case there would possibly be a constant ratio shown between 
their curves of assimilation-maxima. Inspection of fig. 2 shows that this 1s 
not so. The maxima are, at 18° C., in the ratio of 1:1'2, and at 29°°5 C.as 
