ABSORPTION OF CARBON 31 



decomposes in the living plastid it is restored at the expense of 

 other substances. Some suppose that chlorophyll is not in a 

 free state in the plastids but is combined with the protein 

 substances of the stroma, which makes it more stable. Still 

 others assume that chlorophyll is in a colloidal condition, and 

 therefore possesses different properties from those of a true solu- 

 tion. In favor of the last theory is the observation that the col- 

 loidal solutions of chlorophyll, obtained by highly diluting the 

 alcohol solutions with water, or by triturating the leaves with 

 water, are much more stable when exposed to light than the 

 usual solutions in alcohol or benzine. 



10. Quantitative Relations between Absorbed and Stored 

 Energy in the Plant. — Of much importance is the question of 

 how much the leaf profits from the radiant energy which it receives, 

 what part of the latter is stored as potential energy of the accumu- 

 lated organic compounds, and what part is utilized in other proc- 

 esses or given off to the atmosphere. Determinations of the 

 radiant energy which falls on the leaf and which passes through it 

 show that on the average about 75 per cent of it is absorbed, while 

 25 per cent, especially the green and red rays, are either reflected or 

 else transmitted. From this it does not follow, however, that all 

 of the absorbed energy is utilized by the leaf in photosynthesis. 

 The amount of energy that is actually used may be determined in 

 various ways. The total quantity of matter accumulated by the 

 leaf during a certain interval of time may be determined and com- 

 pared with the amount of energy absorbed during this time. The 

 dry- weight method of Sachs, described in Art. 2, may be conveniently 

 used for this purpose. The increase in dry weight per unit leaf 

 area will show, as has been noted, the amount of dry matter accu- 

 mulated. And, knowing the heat of combustion, it is then possible 

 to calculate the percentage of stored energy. A number of valuable 

 determinations were made according to this method by Krashe- 

 ninnikov. Another method is to evaluate the amount of carbon 

 dioxide decomposed by the leaf during a certain period, without 

 injuring the leaf. Since the amount of energy required for this 

 process is known, it is easy to compare it with the total amount of 

 radiant energy which is absorbed by the leaf during a certain 

 period. This method was used by the English scientists Brown 

 and Escombe. The third method, employed by Purievitsch, is 

 based on the fact that a leaf decomposing carbon dioxide absorbs a 



