i 
TRANSACTIONS OF SECTION B. . OFF 
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 leat separately, the increase or 
decrease in the carbon dioxide content of the air being determined by absorption 
and titration in the manner [ have already alluded to. 
In this way we were able to employ comparatively large leaf areas, and to con- 
tinue 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 exist on both the upper and under sides of a leaf 
gaseous exchanges take place through both surfaces, and, as a rule, in some sor 
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 consideration of the ratio of distribution of 
the stomata.! Nevertheless, 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 true 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 brgnonioides is hypostomatic, and therefore takes in carbon 
dioxide only by its lower surface. Under favourable conditions it is quite possible, 
during assimilation, to obtain an intale of atmospheric carbon dioxide into this leat 
at the rate of 700 c.c. per square metre per hour (measured at U° and 76U mm.), 
corresponding to an average linear velocity of the carbon dioxide molecules of 3°8 
centimetres per minute, supposing the intake to be distributed evenly over the 
whole cf the lower leaf surface. This velocity is aimost exactly one-half of that 
at which carbon dioxide will enter a freely exposed surface of a solution of caustic 
alkali. But if the intake of the gas is confined to the stomatic openings of the 
leaf, its velocity of ingress must be very much greater than this. 
We have curefully determined the number of stomata occurring on a given 
area of this particular leat, and also the dimensions of the openings, and find that 
the total area of the openings, supposing them to be dilated to the fullest possible 
extent, amounts to just under one per cent. of the leaf surface. It follows from 
this that the average velocity of the atmospheric carbon dioxide in passing through 
these openings must be 380 centimetres per minute, that is to say, just fifty times 
greater than into a freely exposed absorbent surface of alkali. In other words, 
1 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 appa- 
ratus which governs the size of the openings is so readily influenced, amongst other 
things, by differences of illumination, that @ priori 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 is in 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 to a more rapid diffusion through 
the openings in the upper side of the leaf, ‘ 
