pringsheim's researches on chlorophyll. 97 



tions of ammonia copper sulphate allow a little of the dark 

 green to j)ass, and brighter copper chloride ones give passage 

 to even more blue. The feebly illuminating blue rays appear 

 then more active than the more strongly lighting green ; and 

 this throws an unexpected light on the significance of green 

 colour to vegetation, for the blue rays which are most active 

 in ])roducing oxidation of the cell-contents are almost com- 

 pletely absorbed. 



c. Combustible and Incombustible Constituents of the Cell 

 in Intense Light. — The destructive action of intense light 

 in plant-cells proceeds only in presence of oxygen. Some 

 only of their constituents are oxidised, others, even in the 

 most intense light, are incombustible. One is justified in 

 assuming that amongst the substances in the cells which, 

 by their behaviour in intense light, show great affinity for 

 oxygen, are to be found the special combustible elements of 

 normal respiration in the green tissues of plants, and that 

 those contents which remain unchanged in presence of 

 oxygen in intense light, can in no way under normal light- 

 conditions serve as supporters of respiration. The behaviour 

 in intense light of the larger and well-known formed consti- 

 tuents of cell-contents is easily made out^but more observa- 

 tions are required to determine which among the more 

 minute yet definitely-formed bodies in the protoplasm are 

 stable and which are unstable. 



1. The colouring matter of cells in intense light. — 

 The changes which the colouring matter undergoes are easily 

 Avitnessed. As has already been pointed out, the colour 

 vanishes from green cells in a few minutes when exposed in 

 presence of oxygen to intense light, though this does not 

 occur when the light is red. No new substance is found in 

 the cell as a product of this destruction, nor is there increase 

 of the pre-existing constituents. It may therefore be con- 

 jectured that the colouring matter passes over directly into 

 gaseous product of respiration. The carbonic acid present 

 is not implicated in the destruction of the colouring matter, 

 for in atmospheric air deprived of carbonic acid the de- 

 colorising process proceeds as quickly and energetically as 

 in ordinary atmospheric air containing carbonic acid, whilst 

 in a mixture of carbonic acid and hydrogen all photo- 

 chemical effects are in abeyance (see Section vii, Experiments 

 28 — 31 and 37 — 40). It would appear, therefore, that green 

 cells placed in a mixture (varying quantitatively) of carbonic 

 acid and hydrogen, free from oxygen gas (in which assimila- 

 tion is quite possible), and exposed to the influence of 



VOL. SXII. NEW SER. O 



