pringsheim's researches on chlorophyll. 95 



gas-mixtures. If Spirof/yt^a or Nitella cells are exposed to 

 light of any colour, and the ordinary atmosphere of the gas 

 chamher is replaced by one of hydrogen free from oxygen, or 

 of a mixture of hydrogen and carbonic acid, they show after 

 twenty minutes no change of colour or of their normal 

 character, and may be kept in this condition for weeks 

 provided other detrimental agencies are excluded. Indeed, 

 green and non-green cells which, in presence of oxygen, even 

 in the relatively cold green and blue light, are killed after 

 an exposure of three to five minutes, retnain uninjured, when 

 oxygen is excluded in light of any colour so long as the 

 exposure is not so protracted as to allow of development of 

 hurtful heat effects (see Section vii, Exp. 2nd Series). The 

 temperature obtaining in such cases in the cell-contents, in 

 each chlorophyll-corpuscle and in each plasma-molecule, high 

 as it must be, cannot then be the essential cause of the appear- 

 ances produced. The substitution of hydrogen for the or- 

 dinary air is accompanied by no reducing action causing 

 warming of the object. The light and heat absorption of 

 the chlorophyll-corpuscles and the other constituents of the 

 protoplasm remain unchanged,- and heat-effects and con- 

 (hiction are essentially alike in both atmospheres. If, there- 

 fore, the destruction of the cell-contents takes place in an 

 atmosphere containing oxygen, but is in abeyance when no 

 oxygen is present, those molecular changes, through Avhich 

 dark heat-rays kill the cell at a lower temperature, do 

 not proceed in the protoplasm. The correct explanation of 

 the appearances seems to be that the light influences the 

 relation of the plant to the oxygen of the atmosphere, and 

 that the illuminating rays increase the chemical affinity of 

 the cell-contents for oxygen. The amount of this action 

 is dependent on the colour of the illuminating rays, and 

 increases with their refraction from the red to the blue end 

 of the spectrum. 



The red rays have been shown to have no distinct photo- 

 chemical ettect. This is true, however, only for rays of a 

 very definite position in the spectrum, from the extrenje red 

 to those of a wave length of •00061 mm. (spectrum §, 

 fig. 28). But even under these, after prolonged exposure, 

 or if the cells are very sensitive, small effects are visible, 

 though it may be doubted whether all are purely chemical 

 and some are not thermal. Thus, the sensitive cells of Meso- 

 carpus scalaris, if exposed to red light (spectrum S, in which, 

 however, are traces of violet), are killed in two to three 

 minutes, but the chlorophyll remains uninjured (fig. 21). 

 Now, as in this spectrum, the temperature rises to 45° C. 



