480 
NAT ORE 
[SEPTEMBER 14, 1899 
already touched slightly—the enormous influence which slight 
changes in the carbon dioxide content of the air exert on the 
rate of its ingress into the assimilating leaf. 
With a constant illumination, either in direct sunlight or 
diffuse light, the assimilatory process responds to the least 
variation in the carbon dioxide, and within certain limits, not 
yet clearly defined, the intake of that gas into the leaf follows 
the same rule as the one which holds good with regard to the 
absorption of carbon dioxide by a freely exposed surface of 
a solution of caustic alkali; that is to say, from air containing 
small but variable quantities of carbon dioxide ¢he entake 2s 
directly proportional to the tension of that gas. 
A single experiment will be sufficient to illustrate this. 
A large sunflower leaf, still attached to the plant and exposed 
to a clear northern sky, gave an assimilation rate equal to 
0°331 gram of carbohydrate per square metre per hour, when air 
was passed containing anaverage amount of 2°22 parts of carbon 
dioxide per 10,000. When the experiment was repeated 
under similar conditions of illumination, but with air containing 
14°82 parts of CO, per 10,000, the intake corresponded to an 
hourly formation of 2°409 grams of carbohydrate per square 
metre. The ratio of the tensions of the carbon dioxide in the 
two experiments is I to 6°7, and the assimilatory ratio is I to 
72, so that the increased assimilation is practically proportional 
to the increase in tension of the carbon dioxide. 
Since an increase of carbon dioxide in the atmosphere sur- 
rounding a leaf is followed by increased assimilation even in 
diffuse daylight, it is clear that, under all ordinary conditions of 
illumination, the rays of the right degree of refrangibility for 
producing decomposition of carbon dioxide are largely in excess 
of the power of the leaf to utilise them. Under natural con- 
ditions this excess of radiant energy of the right wave-length 
must, from the point of view of the assimilatory process, be 
wasted, owing to the limitation imposed by the high degree of 
dilution of atmospheric carbon dioxide. But although the 
actual manufacture of new material within the leaf lamina is 
so largely influenced by small variations in the carbon dioxide 
of the air, we are not justified in concluding that the plant as a 
whole will necessarily respond to such changes in atmospheric 
environment, since the complex physiological changes involved 
in metabolism and growth may have become so intimately cor- 
related that the perfect working of the mechanism of the entire 
plant may now only be possible in an atmosphere containing 
about three parts of carbon dioxide in 10,000, 
We have commenced a series of experiments which will, I 
hope, throw considerable light on this point, but the work is 
not at present in a sufficiently advanced state for me to make 
more than a passing allusion to it. 
The penetration of the highly diluted carbon dioxide of the 
atmosphere into the interior air-spaces of the leaf on its way to 
the active centres of assimilation must, in the first instance, be 
a purely physical process, and no explanation of this can be 
accepted which does not conform to the physical properties of 
the gases involved. 
Since there is no mechanism in the leaf capable of producing 
an ebb and flow of gases within the air spaces of the mesophyll 
in any way comparable with the movements of the tidal air in 
the lungs of animals; and since also the arrangement of the 
stomatic openings is such as to effect a rapid equalisation of 
pressure within and without the leaf, we must search for the 
cause of the gaseous exchange, not in any mass movement, but 
in some form of diffusion. This may take place in the form of 
open diffusion through the minute stomatic apertures, which are 
in communication both with the outer air and the intercellular 
spaces, or the gaseous exchange may take place through the 
cuticle and epidermis by a process of gaseous osmosés, similar to 
that which Graham investigated in connection with certain 
colloid septa. 
For many years there has been much controversy as to which 
form of gaseous diffusion is the more active in producing the 
natural interchanges of gases in the leaf. The present occasion is 
not one in which full justice can be done to the large amount of 
experimental work which has from time to time been carried 
out in this direction. Up to comparatively recently the theory 
of cuticular osmosis has been the one which has been more com- 
monly accepted, free diffusion through the open stomata being 
considered quite subsidiary. In 1895, however, Wir: eae 
Blackman brought forward two remarkable papers which opened 
up an entirely new aspect of the subject. After showing the 
NO. 1559, VOL. 60] 
fallacy underlying certain experiments of Boussingault, which 
had been generally supposed to prove the osmotic theory of 
exchange, Mr. Blackman gave the results of his own experiments 
with a new and beautifulty constructed apparatus, which enabled 
him to measure very minute quantities of carbon dioxide given 
off from small areas of the upper and under sides of a re- 
spiring leaf, and also to determine the relative intake of carbon 
dioxide by the two surfaces during assimilation in air artificially 
charged with that gas. The conclusions drawn are that respira- 
tory egress, and assimilatory ingress of carbon dioxide, do not 
occur in the upper side of a leafif this is devoid of stomatic 
openings, and that when these openings exist on both the upper 
and under sides the gaseous exchanges of both physiological pro- 
cesses are directly proportional to the number of stomata on 
equal areas, hence in all probability the exchanges take place 
only through the stomata.! 
These observations of Mr. Blackman are of such far-reaching 
importance, and lead, as we shall see presently, to such remark- 
able conclusions with regard to the rate of diffusion of atmo- 
spheric carbon dioxide, that we felt constrained to inquire into the 
matter further, and for this purpose we employed a modified form 
of the apparatus which we have used throughout our work on as- 
similation. This was so arranged that a current of ordinary air 
could be passed, just as in Mr. Blackman’s experiments, over 
the upper and lower surface of a leaf separately, the increase or 
decrease in the carbon dioxide content of the air being de- 
termined by absorption and titration in the manner I have 
already alluded to. 
In this way we were able to employ comparatively large leaf 
areas, and to continue an experiment for several hours, so that 
we had relatively large amounts of carbon dioxide to deal with, 
and the ratios of gaseous exchange of the two sides of the leaf 
could consequently be determined with considerable accuracy. 
Our results, on the whole, are decidedly confirmatory of Mr. 
Blackman’s observations. The side of a leaf which is devoid of 
stomatic openings certainly neither allows any carbon dioxide to 
escape during respiration, nor does it permit the ingress of that 
gas when the conditions are favourable for assimilation, On 
the other hand, when stomata exists on both the upper and 
under sides of a leaf, gaseous exchanges take place through 
both surfaces, and, as a rule, in some sort of rough proportion 
to the distribution of the openings. There is, however, under 
strong illumination, a greater intake of carbon dioxide through 
the upper surface than would be expected from a mere con- 
sideration of the ratio of distribution of the stomata.? Never- 
theless, the general connection between gaseous exchange and 
distribution of stomata is so well brought out that we must 
regard it as highly probable that these minute openings are the 
ae paths by which the carbon dioxide enters and leaves the 
leaf. 
We must now look at certain physical consequences which 
proceed from this assumption, and see how far they can be 
justified by the known or ascertainable properties of carbon 
dioxide at very low tensions, 
The leaf of Catalpa bignondotdes is hypostomatic, and there- 
fore takes in carbon dioxide only by its lower surface. Under 
1 There is one important fact to be borne in mind when considering how 
far these observations exclude the possibility of cuticular osmosis. In the 
many leaves we have examined, Mr. Escombe and I have found that the 
occurrence of stomata on the upper surface of the leaf is always correlated 
with a much less dense palisade parenchyma. The cuticle and epidermis 
under these conditions are in a much more favourable state to allow carbon 
dioxide to pass into the leaf by osmosis than when the closely-packed 
palisade «cells abut against the epidermis, as they do when this is 1m- 
perforate. 
2 Granted that the stomata constitute the paths of gaseous exchange, it 
is clear that the amount of diffusion through them, other things being 
equal, must depend very largely on the extent to which they are opened. 
The delicate self-regulating apparatus which governs the size of the open- 
ings is so readily influenced, amongst other things, by differences of 
illumination, that a2 /7or2 we should not expect the stomata on the upper 
surface of an insolated leaf to be in the same condition as those of the more 
shaded lower surface. This may very well account for the stomatic ratio 
of the two sides not being in closer correspondence with the assimilatory 
ratios, as found in most of our experiments carried out in bright sunlight. 
In light of lesser intensity there is always a closer correspondence of the 
two ratios. 
There is also another possible explanation of the fact. Since we have 
good reason to believe that the principal part of the assimilatory work is 
carried on by the palisade parenchyma, which occurs in the upper side of 
the leaf, the tension of the carbon dioxide in the air spaces of that part of 
the mesophyll is probably less than it isin the spongy parenchyma. There 
will, therefore, be a higher ‘‘diffusion gradient” between the carbon 
dioxide of the outer and inner air in the former case than in the latter, and 
this would certainly tend toa more rapid diffusion through the openings in 
the upper side of the leaf. 
Ps 
