President's Address. 339 



dition for assiniilation 1 Another theory, ^yhich regards chlorophyll 

 colouring matter as a necessary condition for the decomposition of 

 carbonic acid and water, assumes that it serves as a medium of 

 transfer for the light. It absorbs rays of light which are then 

 passed on to and act upon the protoplasm of the cell, whilst the 

 colouring matter itself remains unchanged. Such an hypothesis 

 would 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 follows from our experience of gas inter- 

 change in plants, 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 tlie reduction process, and it might find a very simple 

 explanation in the absorbed rays being the source of energy. But 

 whea the change in amount of respii-ation in light is borne in mind, 

 and also that the gas interchange in green tissues is always the 

 expression 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 respu-ation is 

 less than assimilation, not that the green colour is an agent in the 

 process. 



Now, as under certain conditions of low light-intensity assimila- 

 tion 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 com- 

 mences 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 assimi- 

 late 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 appearance of green colour in the plant. 



All direct observations fail to confirm the theory that the source 

 of energy for the reduction process lies in the colouring matter. 



