In a culture kept by Meera's class, each Paramecium divides by binary fission about every 8 hours…

Exponential growth: $N = N_0 \times 2^{t/T}$, with doubling time $T$, or $N = N_0 e^{rt}$, with growth rate $r$. They agree when $T = \dfrac{\ln 2}{r}$. In Python: N0 * 2 ** (t / T) and N0 * np.exp(r * t); a loop doubles with N = 2 * N. On a log axis the growth is a straight line.