SEPTEMBER 14. 1899] 
WA TURE 
479 
mated by a double titration process, suggested a few years ago 
by Hart, and we have succeeded in so far improving this method 
that there is no difficulty in determining in 100 c.c. of the 
alkaline solution an amount of carbonate corresponding to ;'5 c.c. 
of carbon dioxide. 
There is practically no limit to the amount of air which can 
be passed through an absorbing apparatus such as I have de- 
scribed, and one of very moderate dimensions will allow from 
100 to 150 litres per hour to pass with perfect safety. Larger 
amounts can be dealt with either by increasing the size of 
the apparatus or by using several smaller ones arranged in 
parallel. 
With proper precautions, determinations can certainly be 
made to within ‘02 part of carbon dioxide in 10,coo of air, so 
that with an apparatus of this kind it is possible to estimate 
the intake of carbon dioxide into a leaf or plant from ordinary 
atmospheric air, and to keep a sufficiently rapid stream of air 
passing over the leaf to maintain the tension of the carbon 
dioxide only slightly below the normal amount. 
The air is measured by carefully standardised meters, reading 
to about 20 c.c. ; and since the amounts of air aspirated vary 
from 100 to goo litres or more, there are practically no errors of 
measurement. The tension at which the air passes through the 
absorption apparatus is measured by a manometer, and all the 
volumes are reduced to standard temperature and pressure. 
All such experiments of course necessitate, not only a deter- 
mination of the carbon dioxide in the air which has passed over 
the leaf or plant, but also a simultaneous determination of the 
carbon dioxide in the ordinary air used. The aecumulation of 
these air determinations clearly shows that the ordinary state- 
ments of our text-books as to the amount of carbon dioxide and 
its limits of variation are altogether misleading. 
In our experiments the air was in all cases taken froma height 
of four feet six inches from the ground, the amounts aspirated 
varying from 100 to 500 litres. 
In the month of July 1898, the minimum amount of carbon 
dioxide found was 2°71 parts per 10,000 of air, and the maximum 
2°86. During the winter months, when the ground was almost 
bare of vegetation, it rose to from 3°00 to 3°23 parts per 10,000 ; 
and on one foggy day, March 16, 1899, after a whole week ot 
similar weather, we found the very exceptional amount of 3°62. 
Asa rule, we may take it that the amount of carbon dioxide in the 
atmosphere during the period of greatest plant growth rarely 
falls short of 2°7 parts per 10,000, and rarely exceeds 3°0 parts, 
with an average of about 2°85. These numbers come very 
close to the determinations of Reiset, and of Miintz and 
Aubin, and agree also fairly well with the Montsouris deter- 
minations. 
If instead of taking the air from a height of three or four 
feet from the ground, we examine the stratum only one or two 
centimetres above the surface of a soil free from vegetation, we 
find, as might be expected, a very large increase in the amount 
of carbon dioxide, which may exceed, under these circumstances, 
12 or 13 parts per 10,000 of air. Such a soil, in fact, gives off 
by diffusion into the surrounding air an amount of carbon 
dioxide which is comparable to that evolved by a respiring 
leaf, that is to say, about 50 c.c. per square metre per hour. 
This is probably a factor which has to be taken into account in 
considering the assimilative power of vegetation of very low 
growing habit, but in all other cases we may assume with 
safety that aérial plants have to take in their carbon dioxide 
from air in which its tension does not exceed ;5%y5 of an 
atmosphere. 
The actual intake of carbon dioxide is determined by en- 
closing the entire leaf in specially constructed air-tight, glazed 
cases, through which a sufficiently rapid air stream is passed. 
These cases are so arranged that the leaf can be enclosed whilst 
still attached to a plant which is growing out in the open under 
perfectly natural conditions, and some of them are sufficiently 
large to take the entire leaf of a sunflower. 
The carbon dioxide content of the air is determined both 
before and after its passage through the apparatus, and since 
the amount of air passed is known we have all the data 
requisite for determining the actual amount retained by the 
leaf. 
An experiment generally lasts from five to six hours, and the 
carbon dioxide fixed in this time may amount to 150 c.c. or 
more, the actual error of determination being very small 
indeed. 
For purposes of comparison the volumes are reduced to the 
NO. 1559, VOL. 60] 
actual number of cubic centimetres of the gas absorbed by a 
square metre of leaf in one hour, which of course necessitates 
an exact determination of the area of the leaf. This is most 
conveniently effected by printing the leaf on sensitised paper, 
and tracing round its outline with a planimeter set to read off 
square centimetres—a far more accurate and expeditious plan 
than that of cutting out a fac-simile of the leaf from paper of a 
known weight per unit of area. 
If it is desired to estimate the assimilative power of a leaf in 
an atmosphere artificially enriched with carbon dioxide, the air 
stream before entering the leaf case is passed through a small 
tower containing fragments of marble, over which there drops 
a very slow stream of dilute acid, whose rate of flow is so pro- 
portioned to the air stream as to give about the desired enrich- 
ment with carbon dioxide. The stream of air is then divided, one 
part going on directly to the leaf case, whilst the other passes 
through a separate absorption apparatus and meter for the 
accurate determination of its carbon dioxide content. 
In order to show the kind of results obtained in this manner, 
I will give one or two examples. 
A leaf of the sunflower, having an area of 617°5 sq. cm., was 
enclosed in its case whilst still attached to the plant, and was 
exposed to the strong diffuse light of a clouded sky for five and a 
half hours, air being passed over it at the rate of nearly 150 litres 
per hour. The content of the air in carbon dioxide as it en- 
tered the apparatus was 2°80 parts p2r 10,000, and this was 
reduced to 1°74 parts per 10,000 during its passage over the leaf. 
This corresponds to a total absorption of 139°95 c.c. of carbon 
dioxide, or to an intake of 412 c.c. per square metre per hour. 
If we assume that the average composition of the carbo- 
hydrates formed is that of a C;H,,O, sugar, the above amount 
of carbon dioxide corresponds to the formation of 0 55 gram of 
carbohydrate per square metre per hour. But we must bear in 
mind that the average tension of the carbon dioxide in the leat 
case was only equal to 1°93 parts per 10,000—that is, only 
about seven-tenths of its tension in the normal air. A cor- 
rection has therefore to be made if we wish to know how much 
the leaf would have taken in, under similar conditions of insola- 
tion, if it had been bathed with a current of air of sufficient 
rapidity to practically keep the amount of carbon dioxide constant 
at its normal amount of 2°8 per 10,000. We shall see later on 
that, well within the limits of this experiment, the intake is pro- 
portional to the tension, so that applying this correction we may 
conclude that under identical conditions of insolation and 
temperature this leaf would have taken in an amount of carbon 
dioxide from the free air at a rate sufficient to produce o°8 
gram of carbohydrate per square metre per hour. This is 
almost exactly equal to the assimilation rate of the sunflower 
which I deduced in 1892 from the indirect process of weighing 
equal areas of the leaf lamina before and after insolation, and it 
also agrees fairly well with some of Sachs’ original experiments 
of a similar nature. 
In another experiment made with the leaf of Catalpa big- 
nonzozdes in full sunlight, the amount of carbon dioxide in the 
air passing over the leaf fell from 2°80 to 1°79 parts per 10,000, 
the total hourly intake for the square metre being 344°8 c.c. 
When this is corrected for tension, it corresponds to an assimil- 
ation in free air of 0°55 gram of carbohydrate per square metre 
per hour. 
An increase in the intensity of the daylight, as might be 
expected, influences to some extent the rate of intake of atmo- 
spheric carbon dioxide; but providing the illumination has 
reached a certain minimum amount, a further increase in the 
radiant energy incident on the leaf does not result in anything 
like a proportional amount of assimilation. We have found, 
for instance, that the rate of assimilation of a sunflower leaf, 
exposed to the clear northern sky on a warm summer's day, 
was about one-half of what it was when the leaf was turned 
round so as to receive the direct rays of the sun almost normal 
to its surface. Now in this latter case the actual radiant energy 
received by the leaf was at least twelve times greater than was 
received from the northern sky, but the assimilation was only 
doubled. 
These differences in the effect of great variation of illumin- 
ation become stil less marked when we use air which has been 
artificially enriched with carbon dioxide. In one instance of 
this kind, for example, we found the assimilation in the full 
diffuse light of the northern sky to be 87 per cent. of what it 
was in direct sunshine. 
This brings me to another interesting point on which I have 
