Carbon Assimilation. 
24 
The fundamental characteristic of the green plant is that 
during its life it produces these compounds of greater energy from 
simple compounds, the energy required for this being obtained from 
the radiant energy of the sun. 
One of the chief elements thus built up into the plant is 
carbon, which is provided by the carbon dioxide of the air. It is 
in this sense, and this only, that we use the term carbon 
assimilation. It is with various aspects of the complex processes 
involved in this that we shall deal in these articles. 
In attempting to summarise the subject, the use of terms such 
as photosynthesis, photolysis of carbon dioxide, etc., are confusing, 
since each of such terms indicates a definite theory of the nature 
of the processes; and our main object is rather to emphasise the 
complexity of the subject, than to suggest or support any single 
theory. 
The processes of carbon assimilation are dependent upon the 
transformation of the sun’s radiant energy into chemical energy. 
Now we know a good deal about the relations between heat, 
electrical and chemical energy, but unfortunately for our purpose 
little is known concerning the relations between radiant energy and 
chemical energy. Yet the connection between radiant and chemical 
energy is most important, for upon it all life depends. 
The very great importance of this aspect of the subject is 
perhaps not completely realised, nor how necessary the development 
of our knowledge of this relationship may be for future generations. 
We may, perhaps, then be justified in giving a few figures to indicate 
its importance. 
It is, of course, owing to the sun’s energy now stored in coal, 
and transformed from radiant to chemical energy by plants ages ago, 
that our present well-developed industrial civilisation is largely due. 
It is interesting then to enquire into the quantity of such energy there 
is available on the earth and the time we are likely to take to use it. 
Naturally, such calculations are rather uncertain; we may 
however quote a recent one due to Gibson. According to this estimate 
the quantity of coal in the earth is 4 x 10 12 tons. In 1905 there were 
mined 9 X IO 8 tons, in 1910 11 x 10 8 tons. If one assumes the 
consumption of coal to increase at this rate, all the coal will be used 
up in about 350 years. But of course long before that period has 
elapsed coal will be difficult to get, and consequently scarce and 
expensive and some other supply of energy will be necessary if an 
industrial civilisation such as ours is to continue. 
