Report on the Bacteriology of Water. I"'- 1 



Passing on from tins point the doubling periods fall to about 100 

 minutes near 16 C., and the shape of the curve now begins to bo 

 apparent (see Curve Gr H) ; about 70 to 80 minutes near 18, 60 to 

 70*minutes near 20, falling to 50 minutes near 22, and about 40 

 minutes near 24 C. to 26 C., 35 to 30 minutes near 2830 C., and 

 this rate of growth may be kept up, for a short time at any rate, at 

 all temperatures thence to 36 or 37 C., but with certain com plica- 

 tions to be referred to shortly. 



It may be inferred from the facts, that growth occurs, at &very slow 

 rate, at some minimum temperature near 5 or 6 C., and that 

 starting just beyond an infinitely long period, the doubling period at 

 the minimum temperature occupies many hundreds of minutes, 

 rapidly shortening for each degree as we raise the temperature to 

 12 15, and falling more slowly at each step as the temperature 

 becomes more favourable. 



It never seems to fall below about thirty minutes, however (the 

 lowest recorded for a post-germinal filament is twenty-seven minutes), 

 and we may therefore assume that at the quickest rate, cell-division 

 demands thirty minutes or so for its completion for it will be con- 

 ceded that the doubling period of a filament is the expression of the 

 time occupied by a complete cell-division and growth, since it is 

 evident from the uniform length of the cells that when the filament 

 is twice as long as at first, it has twice as many cells in it. 



Now the experiments show over and over again that at higher 

 temperatures i.e., temperatures beyond 25 20 C. this minimum 

 doubling period is only approached during the early stages of the 

 growth at the given temperature, and when we reach temperatures 

 near 30 C. and beyond, the filaments occupy fifty or sixty or even . 

 more m nutes to complete a second or third doubling, and after a 

 time fail to complete the period if the high temperature is main- 

 tained. 



lu other words, these high temperatures gradually tell on the 

 power of the organism to maintain its best rate of growth, and the 

 doubling period gets longer and longer the further the temperature 

 is beyond the optimum. The optimum temperature thus comes to be 

 that temperature at which the organism can longest maintain its 

 ability to double its length (or, which is the same thing, complete its 

 cell-divisions) in the shortest time, and so turn the maximum amount 

 of food-materials into cell-substance or, crop. This optimum, tem- 

 perature is 25 C. to 28 C., or very near these, as the curves show. 



The maximum temperature, therefore, is not a fixed point, until 

 we approach 39 to 40 C., beyond which no growth seems possible ; 

 but it differs according to the length of time the organism has been 

 exposed to the high temperature. 



Thus, it frequently occurs that a first doubling period is completed 



