OF LIVING MATTER 31 



tions of life, result from the physical and chemical reactions taking 

 place between the substance of which the living units are composed 

 and the medium in which they exist. 



It is well known in regard to their medium that more or less 

 oxygen always exists in a state of solution in such a fluid, and that 

 the substance of the living units is constantly undergoing oxygena- 

 tion and giving off carbonic dioxide, in addition to or as an integral 

 part of, the other concomitant chemical processes concerned in the 

 assimilation of materials from the medium — whereby its own sub- 

 stance is built up and growth occurs. Verworn has shown that if 

 free oxygen is withdrawn from an infusion the movements of 

 Amoebas and other low organisms gradually cease ; but that, even 

 after many hours, the re-addition of oxygen causes their several 

 movements to recommence. Oxygen acts like this upon all sorts 

 of cytodes and unicellular organisms and as le Dantec says {loc. cit. 

 p. 59), " it is very probable that the majority of the normal move- 

 ments of plastides are due to the constantly changing distribution 

 of oxygen in the infusions." 



It is owing to the fact that light influences these chemical 

 reactions that this agent seems to exert a direct effect upon the 

 movements of many of the lower forms of life, causing them, as the 

 anthropomorphists say, to ' seek ' the light. This phenomenon is 

 seen especially with many Bacteria, with spores of Algae, Diatoms, 

 Desmids, Euglense and other alUed organisms, together with 

 flagellate Monads. The movements in the great majority of cases 

 are towards the source of light — rarely away therefrom, but always 

 in relation with the direction of the luminous radiations. The 

 actual explanation of the movement seems to depend upon a 

 relative exaltation in the activity of the chemical processes on 

 the side of the organism on which the light impinges, as le Dantec 

 shows ; the general effect being, as it were, to draw the organisms 

 towards the source of light. 



Then again food-stuffs, as well as chemical compounds of various 

 kinds, are now known to exercise a remarkable attractive influence 

 upon cytodes and unicellular organisms — varying much in different 

 cases both from the point of view of the nature of the organism 

 and the nature of the compound — but in all cases doubtless depen- 

 dent upon the particular chemical activities capable of being 

 excited in the several kinds of living units. These differences, 

 as well as the different kinds of movement, or the absence of 

 movement, and the different behaviour towards light being, as 



