I30 METABOLISM 



differentiations ; these organisms are plants. The plant world forms a reservoir 

 in which the fugitive solar rays are imprisoned, and are made to subserve 

 certain uses.' 



These reflections of Mayer have been completely confirmed, and we must 

 recognize in carbon-dioxide assimilation the source of the collective organic life of 

 our globe, life which ultimately draws its energy from the sun. Only green plants 

 are able to fix the sunlight in this way, and all non-green parts of such plants 

 as well as all non-green organisms are directly or indirectly dependent on this 

 primary and most important synthesis of organic substance (compare Lect. XV). 



Looked at from this point of view it is certainly of the highest importance 

 to examine to what degree the green cells are able to make use of sunlight, how 

 much of that energy they store and how much of it passes out unused. 



It is possible to arrive at an answer to this question, in the first place, by 

 theoretical calculation. Starting from Boussingault's work as a basis, Pfeffer 

 (1871) assumed that a sq. m. of the leaf surface of Nerium formed starch at the 

 rate of 0-000535 g- per second, and, estimating the heat of combustion of starch 

 as 4,100 calories per g., concluded that 2-2 calories of solar energy is used per 

 sq. m. per second, that is to say, less than i per cent, of the total solar energy 

 (which is, according to Pouillet, about 333 calories per sq. m. per second). 

 If, following Sachs, we reckon that a sq. m. of a leaf of Helianthus produces i-8 g. 

 of starch per sq. m. per hour, this would correspond in round numbers to 

 7,000 calories ; but the amount of energy of the sun per sq. m. per hour is, 

 according to Pouillet, equal to 1,200,000 calories, so that only about o-6 per 

 cent, of the total light energy is employed in assimilation. If we adopt Ad. 

 Mayer's (1897) results we get a somewhat higher percentage, viz. 2-4 per cent., 

 but Mayer made his calculations on the total solar energy for the year. Since 

 a considerable portion of this solar energy falls on the earth's surface at a time 

 when no vegetation is there, more than 2-4 per cent, of the actually available 

 light would be effective. On the other hand, Mayer's estimate of the total 

 annual amount of solar energy at a quarter of a million calories must be too low, 

 for Langley's results give double that number, so that after all corrections have 

 been made the value would be again somewhat under 2-4 per cent, (compare also 

 Brown, 1899). 



[According to the very careful researches of Brown and Escombe (1904), 

 not quite i per cent, of the solar energy which falls on the leaf is employed in 

 carbon-dioxide decomposition.] 



Comparison of these calculations teaches us one thing at least, viz. that 

 only a small fraction of the' total solar energy is gained by the plant in assimi- 

 lation ; how large that fraction is, however, we cannot as yet exactly tell. 

 Detleffsen's (1888) attempts to estimate this amount experimentally is there- 

 fore of the greatest interest. He made observations on the absorption of light 

 in a leaf with the aid of a thermopile, placing the leaf alternately in air con- 

 taining 10 per cent, of carbon-dioxide, and air free from that gas. In the 

 former case, when assimilation was going on, he found that more light was 

 absorbed than when there was no assimilation, and estimated that of the total 

 amount of light 0-9 per cent, in one experiment, 0-3 per cent, in a second, and 

 I -I per cent, in a third was absorbed. We must not place too much reliance 

 on these numbers — for the sources of error are numerous and obvious — but 

 Detleffsen's experiments appear to us to offer an interesting starting-point 

 for further research, if aided by appropriate methods. It is quite possible that 

 no more light is absorbed while assimilation is going on than when it is not, for 

 the light which serves to bring about assimilation may, when assimilation is 

 prevented, be transformed into heat, as in the case also of the not inconsiderable 

 amount absorbed by the dead leaf and by a chlorophyll solution. 



Why it is that decomposition of carbon-dioxide is possible only in the pre- 

 sence of chlorophyll we do not know. It has often been suggested that chloro- 



