Report on the Bacteriology of Water. 461 



condition more like the one in which it enters into the constitution of the 

 living substance than any other organic substance we know or at any 

 rate that we can prepare and use outside the living cell. 



In a certain sense then, it is possibly not absurd to compare the 

 peptonised broth-drop, in which a bacterium cell is suspended, to a 

 sort of inverted living cell in which the peptonised food medium 

 should normally be inside the protoplasmic lining, i.e., in a vacuole. In 

 other words, we might, perhaps, roughly compare it to a vacuolated 

 cell turned inside out, a comparison the more justifiable since the 

 bacterium cell seems to have no obvious vacuole : instead of its assimil- 

 able food solution being clothed by the protoplasm, its protoplasm is 

 clothed by the assimilable food- solution (of course I am neglecting 

 the cell-membrane, and do not wish to posh the analogy too far) in 

 a highly unstable condition. 



The practical aspects of this are also interesting, for it will rarely 

 happen that a bacterium cell, or spore, escapes without organic 

 material clinging to it a matter of importance even if it were shown 

 that the action of the light was entirely confined to promoting oxida- 

 tions at the surface of the cells. 



Taking all into account, therefore, it might perhaps be worth 

 investigation (if methods can be devised) how far some heliotropic 

 effects and retardations of growth in higher plants, and the retarding 

 action of light on growth generally, are due to destructive oxidations 

 in the cell-sap of highly combustible food- materials at, or just prior to, 

 the moment they are ready to be assimilated into the living substance of 

 the protoplasm. 



It might possibly remove some of the difficulties connected with 

 the theory of heliotropism of non-cellular coeloblasts, if the light 

 action occurs in vacuoles next the source of illumination though I 

 confess I see no way out of the difficulties of so-called negative helio- 

 tropism, on this or any other hypothesis. 



It certainly suggests reasons why so many adaptations occur in 

 nature to protect fluids, which presumably contain such substances 

 as 1 have referred to, by colour screens of exactly the kind we should 

 expect to be efficient, as I have already referred to elsewhere. 



An interesting result follows from the fact that the doubling period 

 is simply the visible expression of the doubling in length and biparti- 

 tion of all the cells composing the filament. 



Suppose a filament to be 50 ft. long, and composed of ten cells each 

 5 ft long, and that that filament doubles its length in thirty minutes 

 at a given temperature : then the filament, now 100 p long, consists 

 of twenty cells, each of which has taken thirty minutes to divide and 

 double itself ; from this we can deduce the number of bacilli formed 

 in a given time from the doubling periods, although the individual 

 bacilli are themselves invisible, and when we find a curve like that of 



2 i 2 



