President's Address. 325 



strongly. If a comparison is to be made between the light action of 

 blue and green rays so produced, the blue light solution must not be 

 too dark, for this has led to many errors concerning the energy of 

 colour in assimilation ; and it must also be remembered that the 

 light solutions of ammonia copper sulphate allow a little of the dark 

 green to pass, 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 producing oxidation of 

 the cell-contents are almost completely absorbed. 



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 aflinity for oxygen, 

 are to be found the special combustible elements of normal respira- 

 tion 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 

 constituents 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 Avhich are 

 unstable. 



(a) As to the coluuring matter of cells in intense light. — The 

 changes which the colouring matter undergoes are easily witnessed. 

 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. Xo 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 conjectured that the colouring 

 matter passes over directly into gaseous product of respiration. The 

 carbonic acid present is not implicated in the destruction of tlie 

 colouring matter, for in atmospheric air deprived of carbonic acid 

 the decolorising process proceeds as quickly and energetically as in 

 ordinary atmospheric air containing carbonic acid, whilst in a mix- 

 ture of carbonic acid and hydrogen all photochemical effects are in 

 abeyance. It would appear, therefore, that green cells jjlaced in a 

 mixture (varying quantitatively) of carbonic acid and hydrogen, free 

 from oxygen gas (in which assimilation is quite possible), and ex- 

 posed to the influence of intense light (which in other conditions 

 kills and decolorises them), remain, notwithstanding the carbonic 

 acid present, unchanged and green. This fact, that carbonic acid 



