President's Address. 335 



effect of the colouring matter, has been hitherto entirely overlooked 

 in estimating its physiological importance in respiration. 



As death of the cell is not always accompanied by such striking 

 changes as displacement of the nucleus, cessation of movement, &c., 

 which are seen in Spirogyra, NifeJIa, and Tmdescantm, whilst the 

 slightest colour-change is at once manifest, it is not always easy to 

 detect the proper succession of changes in the cell which mark 

 the difference in sensibility to light between the elements ; but 

 between the chlorophyll colouring matter with its vehicle, and the 

 ground substance and its contained matters this difference is very 

 conspicuous. Hypochlorin disappears more quickly than chloro- 

 phyll colouring matter, and the ground substance of the chlorophyll- 

 corpuscles, after almost momentary exposure, loses all its vital 

 peculiarities, whilst the colour remains still unchanged. 



If, then, a green tissue is more sensitive to light than a not- 

 green one, the cause does not lie in the colour but in the presence 

 of easUy oxidisable assimilation products which arise in the chloro- 

 phyll-corpuscles, and spread thence into the protoplasm. The 

 action of light on the colouring matter is an incidental pheno- 

 menon, and the essential one is the far stronger destruction which 

 the ground substance of the chlorophyll-corpuscles and their in- 

 cluded substances suffer. The clilorophyll-corpuscles are, therefore, 

 extremely sensitive plates having a green screen. The actions 

 stirred up within them by light are both reducing and oxidising, 

 and for the latter their spongy construction and the possession of 

 bodies such as oil and hypochlorin, easily converted into resins, 

 renders them specially well fitted. To the green colouring matter 

 no other share in these processes can be assigned than a physical 

 one due to its colour ; it diminishes the intensity of light, and 

 thereby the amount of oxidation in the cell. This protection, how- 

 ever, fails in intense sunlight, as then the colouring matter is itself 

 destroyed. 



How far extends and wherein consists this protection under 

 normal conditions of plant existence 1 Protection from danger to 

 life of the ceU from intense light has not to be considered. That is 

 never or rarely required by the plant. It is only a protection of 

 assimilation products in the chlorophyll-corpuscles from too rapid 

 destruction in daylight that has to be provided. 



The amount of respiration in gTeen tissues must necessarily, from 

 what has been said, increase in daylight with increasing brightness ; 

 assimilation also rises in amount in light, but nearly reaches its 

 maximum in medium day-brightness. Now, supposing the green 

 tissues could perform their functions without chlorophyll colouring 

 matter, the respiration woiUd, in all intensities of daylight, and 



