3] 



EFFECT UPON GENERAL FUNCTIONS 



175 



hydrogen (Fig. 49). That it is here also the oxygen (and not 

 the carbon dioxide) of the air which is the destructive agent 

 is shown by subjecting the plants to air freed of carbon dioxide, 

 when they are killed by light as before. 



The most important results following from the conclusions 

 of this sub-section are : a minimum vital limit of light action 

 exists only in the case of 

 those organisms (chloro- 

 phyllaceous plants) which 

 depend upon light for as- 

 similation ; a maximum 

 limit is found among the 

 most diverse organisms, 

 those with chlorophyll 

 and those without. The 

 rays which have the more 

 rapid vibrations are the 

 more active. They pro- 

 duce chemical changes to 

 which death is primarily 

 due. 



3. Effect of Light upon 

 the Movement of Proto- 

 plasm. --Under this head 

 we shall consider only 

 those protoplasmic move- 

 ments which may not be 

 grouped under Locomo- 

 tion, and shall discuss 

 three classes of cases : 

 (a) effect of low intensity of light upon movement ; (5) effect 

 of high intensity of light upon movement ; and (c) effect of 

 change of intensity on contraction. 



a. Effect of Low Intensity of Light on Movement-- Dark-rigor. 

 We have already seen that chlorophyllaceous plants must 

 eventually die if kept in the dark. Some time before death 

 occurs the plants go into a condition of immobility, which may 

 be called dark-rigor, since return of light brings a return of 

 movements. Dark-rigor is very marked in the sensitive plant. 



-; 



FIG. 49. Piece of a sprout of Nitella mucronata 

 which was subjected in a gas chamber to a 

 green light in three successive experiments 

 A, B, C. In experiment A the insolation 

 lasted 2 to 3 minutes, the gas chamber be- 

 ing filled with atmospheric air. In experi- 

 ment B the insolation lasted 20 minutes in 

 the presence of hydrogen. In experiment C 

 the insolation occurred again in the presence 

 of atmospheric air and lasted 5 to 6 minutes. 

 (From PRINGSHEIM, '81.) 



