OcToBeR 16, 1902] 
Leaf A. 
Area of leaf 5 ae, SS 
Volume of air passed per hour, 
reduced to normal temperature 
MMGGDKESSULES.0) css), ssa) cee 159°03 litres. 
CO, content of air entering case 2°80 parts per 10,000. 
si 5 leaving case... 1 64 5 ne 
Mean CO, content of air in contact 
743°1 sq. cm. 
with leaf during experiment 2°22 “F a 
CO, absorbed by leaf per hour 18°44 c.c. 
a5 persq. metre perhour 248 2 c.c. 
Leaf B. 
Area of leat soo Make ee 
Volume of air passed per hour, 
reduced to -normal temperature 
and pressure... Beton: 
CO, content of air entering case ... 
a5 a5 leaving case 
Mean CO, content of air in contact 
with leaf during experiment 14°82 nD “9 
CO, absorbed by leaf per hour IS 7 OE: 
oS per sq.metre per hour 1802°8 c.c. 
863°75 sq. cm. 
72°7 litres. 
25°30 parts per 1000 
412 ” ” 
It is manifest that if we wish to determine the relation of the 
partial pressures of carbon dioxide to the rate of intake of that 
gas into the leaf, we must employ the values representing the 
mean carbon dioxide content of the air in contact with the leaf 
during the experiment, which may be taken as the arithmetical 
mean of the composition of the entering and emergent air. In 
the above experiment, we obtain the following relations :— 
Ratio of partial pressures of CO, in A and B, 2°22: 14°82 or 
1: 6°6. 
Ratio of CO, absorbed per sq. metre of leaf A and B in one 
hour, 248°2: 1802 8 = 1: 7°2. 
Thus by increasing the amount of CO, in the air passing over 
the leaf about sevenfold, we have, under similar conditions of 
illumination, increased the photosynthetic power of the leaf by 
a little more than the same amount. 
Experiments of this nature are necessarily limited to com- 
paratively short periods, and give no information as to how far 
the plant, as a whole, will respond to such changes in its atmo- 
spheric environment. When first drawing attention to these 
facts in 1899 (Presidential Address, British Association, Section 
B, Dover), it was pointed out that we were not justified. with- 
out direct experiment, in concluding that the plant would be 
able to avail itself indefinitely of the increased amount of plastic 
carbohydrate material formed in its leaves under these artificial 
conditions, and that translocation, metabolism and growth may 
have become so intimately correlated that the perfect working of 
the entire plant may only be possible in an atmosphere contain- 
ing the normal amount of three parts of CO, per 10,000, 
Experiments were started to test how far slightly increased 
amounts of CO, in the air would affect the dry weight of plants 
grown in such atmospheres, and they indicate that the plants 
were certainly not stimulated to increased growth by somewhat 
increasing the amount of CO, in the surrounding air. The 
evidence, in fact, points in the other direction, z.e. towards a 
slight diminution in the increment of dry weight, and to a less 
development of foliar area. There were also indications of 
certain morphological differences, which assumed some im- 
portance in the light of subsequent experiments. The plants 
grown in air slightly enriched with CO, had not only smaller 
leaves than the controls, but these leaves were of a distinctly 
darker green, and the internodes of the plants were decidedly 
shorter. 
The results obtained with these preliminary experiments now 
induced the authors to extend their observations to a larger 
number of species, and arrangements were consequently made 
to carry out a series of experiments ona large scale. This was 
done in a greenhouse divided in two by a glazed partition. In 
one half, the plants, gourds, balsams, fuchsias, begonias, &c., 
were exposed to the CO,-laden atmosphere ; in the other, con- 
trol plants were in ordinary air. The original paper must be 
consulted for experimental details and précautions. 
A careful record was made of the differences in appearance of 
the two sets of plants on June 10, June 29and July 13, that 
is to say, at 28, 47 and 61 days from thecommencement of the 
experiment. The results are given in detail in the appendix to 
this paper, and may be summarised as follows :— 
The effect of an increased amount of CO, in the air becomes 
NO. 1720, VoL. 66] 
NATURE 
621 
in most cases apparent within a week or ten days from the com- 
mencement of the experiment, and rapidly increases as time 
goes on. There is a marked difference induced in the habit 
and general appearance of most of the plants owing to a stimula- 
tion of all axial growth, accompanied by a more or less pro- 
nounced shortening and thickening of the internodes. Usually, 
but not in all cases, there is an increased number of the inter- 
nodes, so that the height of the two contrasted sets remains 
much about the same, but the chief difference of general habit 
is brought about by the development throughout the plant of 
secondary axes in the axils of the leaves, thus giving the plants 
grown under the influence of increased CO, a denser and more 
bushy appearance. This was particularly noticeable in the 
fuchsias, especially the dark-leaved variety, in which every axil 
bore a shoot and frequently extra axillary ones. Adventitious 
shoots were also developed rather freely at the base of the plants. 
The leaf area of the plants under the influence of increased 
CO, was generally found to be much reduced, not so much by 
the formation of asmaller number of leaves as by the reduction in 
area of the individual leaves. This was found to be extreme in 
the case of the dark-leaved fuchsias, and it was also very marked 
in the second crop of the leaves of Impatiens. There was also 
produced in many of the plants a marked inward curling of the 
leaves, the extremes in this direction being found in the begonias 
and fuchsias. In the dark-leaved variety of fuchsia, the leaves 
were curled inwards like a watch-spring, which would doubtless 
tend to reduce excessive photosynthesis by preventing the 
normal amount of light from reaching the chloroplasts. This 
change of habit may, in fact, be regarded as an attempt on the 
part of the plant to adapt itself to its abnormal atmospheric 
surroundings. 
The extra CO, in several cases induced a deeper green colour 
in the leaf, and in all other parts of the plant where chlorophyll 
was present. This was particularly noticeable in the second 
crop of leaves developed on the Impatiens, in the begonias and 
in the darker-leaved iuchsias. 
On July 19, the Sachs test for starch was applied to the leaves 
of the two varieties of fuchsia, Cucurbita Pepo and Jmpatiens 
platypetala, In _all cases, the leaves taken from the plants grown 
with increased CO, in the air showed a much larger accumulation 
of starch than did the leaves of the control plants. These 
differences were the most strongly marked in the leaves of 
Impatiens, which became quite black with the test. 
It was, however, in the development of the reproductive 
organs of the two sets of plants that the most striking and im- 
portant differences were found. Whilst the control plants in 
ordinary air flowered, and in some cases fruited luxuriantly, in the 
corresponding plants submitted to air containing 11°4 parts per 
10,000 of CO,, zzflorescence was almost totally inhibited. With. 
the exception of one or two sickly-looking flowers on the 
begonias, not a single flower-bud opened on any of the plants of 
this set. The plants of Impatiens, Kalanchoé and of the 
darker-leaved fuchsias did not even produce a flower-bud, 
whilst in the Nicotiana, Cucurbitas and lighter-leaved fuchsias, 
the small flower-buds which commenced to form were coin- 
pletely shed long before the time of opening. 
In another series of experiments, carried out on similar lines, 
the air of compartment B was enriched with carbon dioxide to 
the extent of 6 per cent., that is to say, up to about 200 times 
the normal amount. The experiment extended from June 3 to 
August 26, and the general results both in the direction and 
amount of change of habit induced in the plants were so 
very similar to those induced in the plants with only three- 
and-a-half times the normal amount of CO, as to require no 
further special description. The results are, however, valuable 
as indicating that the observed differences cannot be due to any 
direct poisonous influence of the carbon dioxide, otherwise we 
should certainly expect a marked difference to be produced by 
increasing the amount of CO, from 11°4 parts per 10,000 to 600 
parts per 10,000, z.e. more than fiftyfold, which was not the 
case to any appreciable extent. 
The direction in which we must search. for the true explana- 
tion of the effect is probably indicated by the experiments on 
leaves described in the early part of the paper, where it was 
shown that the amount of photosynthesis in the leaf lamina is, 
within certain ill-defined limits, a function of the partial pressure 
of the CQ, in the surrounding air. 
In the first series of experiments in the greenhouse, where 
this partial pressure was maintained at about three-and-a-half 
times the normal, the plants for a certain limited period must 
