384 
McLean. — Field Studies of the Carbon 
relation to either the minima or maxima of carbon dioxide absorption. 
Thus the changes in water content of the leaves did not appear to exert 
a controlling influence upon the rate of photosynthesis. 
Other factors of the environment which have been shown to influence 
the rate of photosynthesis of green leaves are : carbon dioxide supply from 
the air, angle of incidence of the suns rays, intensity and composition of 
sunlight. The carbon dioxide supplied to the plants during each test period 
is shown by the difference in titration numbers of the control Pettenkofer 
tube and the control wash bottle. These values are shown below in 
Table VI. 
Table VI. 
Changes in carbon dioxide supply during the tests of coco-nut leaves, as 
indicated by the difference in titration numbers of control Pettenkofer 
tubes and control wash bottles. 
Leaf No . 1. Leaf No . 2. Leaf No . 3. Leaf No . 4. Leaf No . 6. 
Tilrd - Titra - Titra - ' J ' itra - Titra • 
Time 
tion 
Time 
tion 
Time 
tion 
Time 
tion 
Time 
tion 1 
period . 
diff . c . c . 
of acid . 
period . 
diff . c . c . 
of acid . 
period . 
diff . c c . 
of acid . 
period . 
diff . c . c . 
of acid . 
period . 
diff . cA 
of acid , j 
5 - 5 °- 7 - 5 ° 
2-2 
5.30- 6.30 
i-8 
5 - 30 - 7-30 
1*7 
5.30-6.30 
i-6 
5.30-6.30 
2-0 ! 
7.50- 9.50 
1-4 
6.30- 8.30 
1.85 
7.30- 9.30 
1 "4 
8.30 
i-6 
8.30 
1-25 ! 
9.50-1 1.50 
1*3 
8.30-10.30 
i*8o 
9.30-1 1.30 
i *35 
10.30 
i-6 
10.30 
1-15 
11.50- 1.50 
i *5 
10.30-12.30 
i*8o 
11.30- 1.30 
T '35 
12.30 
I *7 
12.30 
1.50- 3.50 
1, 45 
12.30- 2.30 
1*85 
1.30- 3 30 
V °5 
2.30 
J '45 
2.30 
1*10 
3.50- 5.50 
i *3 
2.30- 4-30 
i*8o 
3 * 30 - 5-30 
i*i -5 
4-30 
i *5 
4.30 
1. 10 
5.50- 6.50 
o*6 
4.30- 6.30 
J -75 
5.30- 6.30 
o -75 
6.30 
i -55 
6.30 
1 15 
It will be noted that in all cases the carbon dioxide supply per hour is 
greater at the first hour than for the succeeding hours, and in most cases is 
nearly twice as great. 
It is probable that the carbon dioxide content of the air is actually 
higher at night and in the early morning, due to respiration from the dense 
vegetation, and that this amount decreases rapidly when the plants begin to 
absorb carbon dioxide actively in the morning. 
The high rates of carbon dioxide absorption by the coco-nut leaves in 
the early morning may then be partly due to the presence of more carbon 
dioxide in the air at that time than later. 
That this is not necessarily the most important factor is indicated by 
leaf 2, which absorbed carbon dioxide at a slower rate at 5.30 to 6.30 a.m. 
than at 6.30 to 8.30 a.m., in spite of the fact that the amount of carbon 
dioxide supplied from the air at 5.30 to 6.30 a.m., as indicated in Table VI, 
was nearly as great for that one hour (i*8 c.c. index) as for the two succeed- 
ing hours (1-85 c.c. index). 
The second maximum in the afternoon cannot be explained by changes 
in carbon dioxide content of the air, since it occurs at a time when the latter * 
is markedly low. A more reasonable hypothesis to account for the depres- 
sion in rate of carbon dioxide absorption and subsequent increase in the 
