676 REPoRT—1899, 
necessarily respond to such changes in atmospheric environment, sin¢e the complex 
physiological changes involved in metabolism and growth may have become go inti- 
mately correlated that the perfect working of the mechanism of the entire plant 
mny now ouly 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 con- 
siderable 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 com- 
munication 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 
osmosis, 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 commonly accepted, free diffusion through the open 
stomata being considered quite subsidiary. In 1895, however, Mr. F. F. Black- 
man brought forward two remarkable papers which opened up an entirely new 
aspect of the subject. After showing the 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 beautifully 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 respiring 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 respiratory egress, and assimilatory ingress of carbon 
dioxide, do not occur in the upper side of a leaf if this is devoid of stomatic open- 
ings, and that when these openings exist on both the upper and under sides the 
gaseous exchanges of both physiological processes 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 remarkable conclusions with regard to the 
rate of diffusion of atmospheric 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 assimilation. This was so 
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 
imperforate. 
