44 PHYSICS OF STREAMING 



and the escaping sap neither produces any colouration in reduced indigo- 

 solution, nor excites any movement in motile aerobic bacteria. 



Nor does the plasma contain any occluded oxygen, and the amount of 

 dissolved oxygen it holds is relatively small, even in the immediate neigh- 

 bourhood of actively assimilating chloroplastids. The oxygen produced 

 by them is, in fact, liberated almost immediately upon the external surface, 

 and hence it arises that streaming may ultimately cease in the more aerobic 

 species of Char a (C.fiexilis^ C. stelligera) as well as in strips of leaf-cells of 

 Elodea and Vallisneria after prolonged immersal in a stream of hydrogen 

 containing a little carbon dioxide, even though exposed to continuous 

 illumination. 



It is certain, therefore, that several species of Chara and Nitella are 

 facultative anaerobes, and it is interesting to notice that within certain 

 limits the previous, as well as the immediate external conditions, exercise 

 an important influence upon the degree of anaerobism. Thus this pecu- 

 liarity is more marked in plants from muddy, stagnant water, than in 

 those from clear well-aerated habitats. This not only applies to different 

 species, but also in a less degree to different individuals of the same species. 

 Moreover, the plants are more aerobic at higher temperatures (25 to 30 C.) 

 than at lower ones (15 to ao C.). The facultative anaerobism of Chara 

 and Nitella is to be regarded as a secondary modification produced by the 

 exigencies of their surroundings, and a small fraction of the energy of 

 anaerobic respiration suffices for the maintenance of slow streaming. 



SECTION 15. Influence of Electrolytic Oxygen. 



The action is similar to that of ordinary oxygen, except that an 

 energetic oxidizing action is exercised at the moment of liberation. This 

 may suffice to produce a brownish colouration in the cell-wall, or to deco- 

 lorize coloured cell-sap, or to kill the protoplasm. With weak currents, 

 however, no pronounced injurious effect is at first exercised. Thus, if 

 leaves of Elodea or leaf-sections of Vallisneria are laid across platinum 

 electrodes, covered, ringed, kept in darkness until streaming has ceased, 

 and then traversed by a weak constant current for one to a few minutes, 

 on lifting the dark cover streaming will usually be seen over the positive 

 electrodes, but will not appear over the rest of the leaf until after fifteen 

 to thirty minutes, if the preparations have been kept two to three hours in 

 darkness, so as to temporarily inhibit the resumption of photosynthesis. 

 Similarly, if cells of Chara or Nitella are used, streaming may be distinctly 

 more rapid at the end lying near to the positive electrode, although 

 a strength of current sufficient to produce distinct electrolysis usually acts 

 as a supra-maximal stimulus to streaming in these plants. Sufficiently 

 resistant cells can usually be found after a few trials, if a current of 2 to 3 



