Carbon Assimilation. 
243 
Part of the work of the plant physiologist must be to get a clear 
insight into the processes by which sun energy is stored up, not 
merely that by so doing plant cultivation shall be made as efficient 
as possible and agriculture made a real plant industry, but also that 
by clearing up these problems in the plant, a way may be opened for 
allied sciences, pure and applied, to find methods by which it will 
be possible to utilise or transform sun energy directly. 
When aether vibrations in the form of light pass through matter 
the system can become affected in different ways. 
1. The temperature of the body may be raised: the light energy 
is converted into heat. 
2. The vibrations may give rise to changes of a chemical 
nature. 
The first of these is a very general phenomenon. Most 
substances can change the energy of aether vibrations into heat. 
The extent to which this takes place depends on the nature of the 
substance, the thickness of the absorbing layer, and the wave-length 
of the light. 
On the other hand, the chemical effect of light takes place only 
in exceptional cases. These constitute the so-called photochemical 
reactions. They are of the greatest interest, for they involve the 
conversion of aether vibrations into chemical energy. 
Only during the last ten or fifteen years have investigations 
told us something about these photochemical reactions, and in spite 
of much uncertainty several facts have now become definite. Our 
intention is to draw attention to these facts, which deserve to be 
more generally known by plant physiologists than is generally the 
case. Above all we want to emphasise the fact that transformation 
of light energy into heat energy and of light energy into chemical 
energy are two essentially different phenomena. Although in the 
organisation of the plant these two phenomena may be connected and 
inter related, yet the basis of our enquiries into carbon-assimilation 
should be that carbon assimilation consists of photochemical 
reactions, the main feature of which is that radiant energy from 
the sun, by means of chlorophyll in the plant, is transformed to 
chemical energy. 
We propose to commence by giving an account of the main 
facts that recent investigations have made clear about the pigments 
in the plant, before considering the processes which may possibly 
occur and the factors influencing those processes. 
