110 pringsheim's researches on chlorophyll. 



Tradescantia, 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 sensi- 

 hility to light between the elements ; but between the chloro- 

 phyll colouring matter with its vehicle, and the ground- 

 substance and its contained matters this difference is very con- 

 spicuous. 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 easily-oxidisable assimilation-products which 

 arise in the chlort)phyll-carpuscles, and spread thence into 

 the protoplasm. Tlie action of light on the colouring matter 

 is an incidental phpenomenoii, and the essential one is the 

 far stronger destruction which the ground-substance of the 

 chlorophyll-corpuscles and their included substances suffer. 

 The chlorophyll-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 <linriinishes 

 the intensity of light, and thereby the amount of oxidation 

 in the cell. This protection, however, fails in intense sun- 

 light, as then the colouring matter is itself destroyed. 



How far extends and wherein consists this protection 

 under normal conditions of plant existence ? Protection 

 from danger to life of the cell 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 chlo- 

 rophyll-corpuscles from too ra{>id destruction in daylight 

 that has to be provided. 



The amount of respiration in green tissues must neces- 

 sarily from what has been said increase in daylight with in- 

 creasing brightness ; assimilation also rises in amount in 

 lio-ht, but nearly reaches its maximum in medium day- 

 brightness. Now, supposing the green tissues could perform 

 their functions without chlorophyll colouring matter, the 

 respiration would, in all intensities of daylight, and espe- 

 cially in the brighter light, greatly exceed in amount the 

 assimilation (fig. 29 c, d). An accumulation of carbon would 



