s in fig. 4. The abscissae
represent intervals of time, the ordinates the measured lengths of the
growing filament. Thus, at 2.33 P.M. the length of the filament was 6
[micron]; at 5.45, 20 [micron]; at 8 P.M., 70 [micron] and so on. Such
curves show differences of steepness according to the temperature (see
temp. curve), and to alterations of light (lamp) and darkness. (H. M. W.)]

[Sidenote: Measurement of growth.]

The growth of an ordinary bacterium consists in uniform elongation of the
rodlet until its length is doubled, followed by division by a median
septum, then by the simultaneous doubling in length of each daughter cell,
again followed by the median division, and so on (figs. 13, 14). If the
cells remain connected the resulting filament repeats these processes of
elongation and subsequent division uniformly so long as the conditions are
maintained, and very accurate measurements have been obtained on such a
form, _e.g._ _B. ramosus_. If a rodlet in a hanging drop of nutrient
gelatine is fixed under the microscope and kept at constant temperature, a
curve of growth can be obtained recording the behaviour during many hours
or days. The measured lengths are marked off on ordinates erected on an
abscissa, along which the times are noted. The curve obtained on joining
the former points then brings out a number of facts, foremost among which
are (1) that as long as the conditions remain constant the doubling
periods--_i.e._ the times taken by any portion of the filament to double
its length--are constant, because each cell is equally active along the
whole length; (2) there are optimum, minimum and maximum temperatures,
other conditions remaining constant, at which growth begins, runs at its
best and is soon exhausted, respectively; (3) that the most rapid
cell-division and maximum growth do not necessarily accord with the best
conditions for the life of the organism; and (4) that any sudden alteration
of temperature brings about a check, though a slow rise may accelerate
growth (fig. 8). It was also shown that exposure to light, dilution or
exhaustion of the food-media, the presence of traces of poisons or
metabolic products check growth or even bring it to a standstill; and the
death or injury of any single cell in the filamentous series shows its
effect on the curve by lengthening the doubling period, because its
potential progeny have been put out of play. Hardy has shown that such a
destruction of part of the