1905.| On the Physiological Processes of Green Leaves. 41 
air by p’, the rate of assimilation in ordinary air by a, and that in enriched 
air by a’; then, within certain limits, p/a = p’/a’. 
Our experiments have shown that this relation of partial pressure to 
assimilation certainly holds good up to 15 parts of carbon dioxide per 10,000 
of air, that is to say, up to a concentration of five times the normal amount ; 
but it is highly probable that this by no means expresses the upward limit of 
the application of this rule, which is one of considerable practical importance 
when we desire to determine the true rate of assimilation in ordinary air, as 
will be seen from the following considerations. 
The actual amount of carbon dioxide which has been abstracted from a 
measured volume of air passed over the leaf is expressed by the difference in 
the amount of carbon dioxide in the air on entering and emerging from the 
leaf-case, and since the mean carbon dioxide-content of the air in contact 
with the leaf must necessarily be less than that of the outer air, a correction 
must be made if we desire to ascertain the true rate at which the leaf would 
assimilate under free-air conditions, all other conditions being constant. The 
above rule enables us to make such a correction, the magnitude of which 
will be readily seen from the following concrete example taken from an 
actual experiment. 
A leaf of Catalpa bignoinioides having an area of 157:45 sq. em. was enclosed 
in its glazed case and submitted to intermittent sunshine for six hours, 
during which time 322°57 litres of air were passed over it. From an analysis 
of the ingoing and outgoing air it was found that the leaf had abstracted 
32°58 c.c. of carbon dioxide (at 0° and 760 mm.), which is equivalent to an 
assimilation of 3°448 c.c. per square decimetre per hour. Since, however, the 
outer air as it entered the leaf-case contained 2:80 volumes of carbon dioxide 
per 10,000 volumes of air, whilst the emerging air contained only 1°79 volumes, 
the mean carbon dioxide-content of the air in contact with the leaf was only 
ae 179. 2°295 per 10,000. Hence, from the above rule of partial 
pressures, the amount of carbon dioxide which the leaf would have assimilated 
in the outer air, all other conditions being the same, is eae = 4206 cc. 
of carbon dioxide per square decimetre per hour. 
The correction in this case, therefore, which is a somewhat extreme one, is 
+ 22 per cent. 
Although such a method as we are considering ought to give, with a fair 
approximation to accuracy, a measure of the effective assimilation going on in 
the leaf, the results are subject to a further correction if we desire to know 
the exact amount of photosynthetic work accomplished, since there is active 
