PlilNGSHEIBl's RESEARCHES ON CHLOROPHYLL. 115 



are then passed on to and act upon the protoplasm of the 

 cell^ whilst the colouring matter itself remains unchanged. 

 Such an hypothesis Avould be admissible only if it could be 

 otherwise proved that the colouring matter takes part in the 

 destruction of carbonic acid. But this by no means follow- 

 from our experience of gas-interchange in jdants, and our 

 knowledge of assimilation gives no support to such a theory^ 

 for the rays so strongly absorbed by the chlorophyll colouring- 

 matter are of no effect in assimilation. 



That green tissues alone exhale oxygen in light may, at 

 first sight, appear weighty evidence in favour of the colouring 

 matter taking a direct part in the reduction-process^ and it 

 might find a very simple explanation in the absorbed rays 

 being the source of energy. But when the change in 

 amount of respiration in light is borne in mind, and also that 

 the gas-interchange in green tissues is always the expres- 

 sion of the difference between assimilation and respiration, 

 such an explanation is not satisfactory. The exhalation of 

 oxygen by green tissues alone merely proves that in them 

 respiration is less than assimilation, not that the green colour 

 is an agent in the process. 



Now, as under certain conditions of low light-intensity 

 assimilation can proceed without any oxygen being exhaled, 

 it is necessary to consider all conditions under which gas- 

 interchange is taking place before measuring the amount of 

 assimilation by the oxygen exhaled, or considering that the 

 latter is always an index of the extent of the former. The 

 old notion that assimilation only commences when there is 

 a certain degree of brightness of illumination, because under 

 some conditions of low light-intensity no oxygen is exhaled, 

 is incorrect, as has been shown, and assimilation actually 

 goes on at all, even the lowest intensities, but in the lower 

 ones it is concealed by respiration. So that the exhalation 

 of oxygen is only recognisable when respiration is less than 

 assimilation. 



It has been hitherto supposed that a plant only begins to 

 assimilate when it becomes green, it is first green and then 

 assimilates. But this idea has resulted again from the 

 misconception that oxygen must be exhaled if there is 

 assimilation. All that the facts warrant is, that respiration 

 is less than assimilation only when the tissue is green, and 

 consequently the exhalation of oxygen only commences with 

 the appearatice of green colour in the plant, ^ 



All direct observations fail to confirm the theory that the 



' Boussingault (' Comptes Rendus,' torn. 68, p. 410) has shown this is 

 true from the very earliest appearance of colouring matter. 



