
Amount of Carbon Diomde in the Air of Kew. 119 

number of the 91 experiments recorded, in only nine did the carbon dioxide 
slightly exceed 3-20 parts per 10,000. 
As a general rule it would appear that the carbon dioxide present in the 
air is somewhat greater in the winter than in the summer months, but the 
year 1901 was an exception in this respect, since during July of that year 
it averaged 3:11 parts per 10,000. On the whole we are inclined to regard 
the periods of maximum carbon dioxide as being dependent more on anti- 
cyclonic than on seasonal conditions. The variations in the atmospheric 
carbon dioxide are small when regarded as absolute amounts, but they assume 
considerable importance in the light of what we now know about the relation 
of the partial pressure of the carbon dioxide in the air to the actual rate of 
assimilation by the green organs of plants. | 
In a previous communivation* we have shown that under a given set of 
conditions favourable to photosynthesis a living chlorophyllous leaf assimilates 
from the surrounding air in a given time amounts of carbon dioxide which are 
directly proportional to the partial pressure of that gas. This is true up to 
limits of concentration of the carbon dioxide which lie altogether outside the 
variations which occur in Nature. Hence we have reason to expect that any 
well-marked variations in the carbon dioxide-content of the air during the 
active period of plant growth would produce some effect on the rate of 
nutrition of the plant.f 
As a guide to the magnitude of the effects which may be expected from 
such variations as we are considering, we will take the mean carbon dioxide- 
content of the air for the month of July for each of the four years 1898 
to 1901 
* ‘Roy. Soc. Proe.,’ vol. 70 (1902), p. 398. 
+ This argument is not affected by the conclusion arrived at in a former paper (‘ Roy. Soc. 
Proc.,’ vol. 70, p. 397) that although a proportionately increased intake of carbon dioxide takes. 
place in a leaf when surrounded by an atmosphere containing three to four times the normal 
amount of carbon dioxide, yet the plant as a whole cannot avail itself of this increased amount of 
newly synthesized plastic material, owing to defective correlation of its various functions under 
these artificial conditions. Although a want of perfect adaptation of the plant in these respects. 
undoubtedly exists in an atmosphere containing three or four times the normal amount of carbon 
dioxide, it by no means follows that the plant as a whole will not respond perfectly to the slight 
differences of carbon dioxide-content which occur under natural conditions. 
