Carbon Assimilation. 
41 
measurement of energy satisfactory, and on the other hand the 
methods used for measuring assimilation are very crude. Also the 
fundamental aspects of the problem seem to have escaped the 
notice of most investigators, in spite of its vital importance. 
Having regard to the present state of our knowledge concerning 
radiant energy derived from pure physics, and to what we now know 
of the processes of carbon assimilation, it ought to be possible to 
attack the problems profitably. 1 
Concerning the subject with which we deal in this section, 
there exists a very voluminous literature, to which undue importance 
is generally given in text-books. 
The earliest investigators, as for instance, Senebier (1785) and 
Dumas (1824) supposed that the blue-violet rays were of most 
importance in assimilation. Daubeny (1836) and Draper (1844) as 
well as Sachs (1864) and Pfeffer (1871) were of the opinion that the 
yellow rays were those utilised in assimilation. Lommel in 1871 
suggested that the rays most strongly absorbed by chlorophyll, 
namely those between the B and C lines in the red part of the 
spectrum, were those most active in assimilation. 
Attempts have also been made by Timiriazeff (1877, 1885), 
Reinke (1884) and Engelmann (1882, 1884) to discover in which part 
of the spectrum assimilation takes place. They all agree that 
maximum assimilation takes place in the red part of the spectrum, 
although they differ as to the exact position. Engelmann, using the 
most sensitive, though not necessarily the most accurate method, 
obtains a secondary maximum in the blue-violet end of the spectrum. 
Timiriazeff and Richter (1902) appear to be aware of the fact that 
smaller assimilation in the blue part of the spectrum of sunlight 
may be explained by the less intensity of radiation in that region. 
Richter and others contend that the assimilation depends only on the 
energy of the absorbed light and not on the wave length. 
The conditions in all these experiments were such that 
discussion of them is not justified; we mention them here mainly 
on account of their historical interest. 
It does not appear from all this earlier work whether the 
assimilation would be the same if a leaf were exposed to red or to 
1 We are confronted with the following problems:— 
1. The intensities and relative proportions of the different frequencies of 
radiation incident on the leaf. If sunlight is concerned, astrophysical and 
meteorological factors are of considerable influence here, but a discussion of 
this aspect of radiation is outside the scope of this review. 
2. The relative absorption by the leaf of radiation of different frequency. 
3. The relative proportion of absorbed energy of any particular frequency 
which is used in assimilation. 
