386 McLean . — Field Studies of the Carbon 
afternoon is the decrease in the number of sun’s rays striking each pinna at 
noon. The pinnae were placed with their long axes north and south in 
each test. The pinna is in cross-section like an inverted V. The incident 
rays strike one half vertically in the early morning, and the other half 
similarly in the afternoon. At noon, the sun’s rays strike both sides 
obliquely, and also the actual area of the horizontal projection of each pinna 
is less than the area of one side of the pinna. Thus the low rate of carbon 
dioxide absorption at midday in these tests may be partly due to their 
position with respect to the sun’s rays at that time. Since, however, the 
sugar-cane leaf, which lies flat and does not decrease its exposed surface at 
midday except by shrinkage, shows the same character of depression in rate of 
carbon dioxide absorption, it is necessary to assume, in this one case of sugar- 
cane at least, that the midday depression is due to a change in the internal 
condition of the leaf itself. 1 
Test Series 7. 
On May 29 two pinnae were detached from leaf 3 of a coco-nut tree 
growing near the one used in previous tests, and were placed in water. 
Then their rate of carbon dioxide absorption was tested for comparison 
with the attached leaves. The data obtained from these tests are presented 
in Table VII and Diagram 7, which are arranged similarly to the correspond- 
ing data in series 2 to 6 (Table V and Diagrams 2 to 6). 
Table VII. 
Absorption of carbon dioxide by two detached coco-nut pinnae of 595*6 
sq. cm. area, during 13 hours on May 29, 1918. 
Difference in 
concentration of 
CO 2 absorbed 
Duration of 
Da{ OH ) 2 from test 
C 0 3 absorbed 
per sq. m. per 
Remarks. 
Test. 
and blank in c.c. 
of 0*0405 normal 
per sq. m. 
hour. 
HCl . 
mg. 
mg. 
5.30- 6.30 a.m. 
0*15 
27 
2 7 
Sun rose at 5.45. 
6.30- 8.30 „ 
o*45 
81 
40 
Struck leaf at 6.10 
8.30-10.30 „ 
o*35 
63 
31 
a.m. Bright sunshine 
30.30 a.m,-i2.30 p.m. 
o*75 
134 
67 
from 6.10 a.m. to 
32.30-2.30 pm. 
o*45 
81 
4° 
1.30 p.m. 
2.30-4.30 ,, 
0*5° 
90 
45 
Light dull from 1.30 
4.30-6.30 „ 
0-30 
54 
27 
to 2.06. Rain 2.06- 
2.50 p m. ; dull 2.50- 
530 mg. 
5.15 p.m. Bright 
sunshine from 5.15 
p.m. to 6.10 p.m , 
when the sun set 
behind clouds. 
The total amount of carbon dioxide absorbed per sq. m, by the 
detached pinnae, 530 mg., is thus almost the same as the amount absorbed 
1 For a report of a similar midday checking in photosynthesis of red clover, see Bot. Gaz., xxvi 
(1898), 347, The original text of this was not available at the time that the present paper was 
prepared. 
