To get insights into the dynamics underlying the first hours of isothermal self-assembly, we used fluorescence microscopy and automated image analysis to establish the length distribution of nanotubes adsorbed on glass slides at different assembly times (Figure ). In TANa buffer (Figure ), the average nanotube length progressively increased with time, reaching up to 5.7 ± 0.35 µm at 24 h (Figure , S3) accompanied by a progressive shift of the distribution (Fig 3C ). To get a clearer picture of how monomers are distributed in the tubes, we also plotted the fraction of the total measured nanotube length occupied by each size range (Figure ). These length fraction plots show that the contribution of short nanotubes diminishes over time, as longer ones dominate, consistent with what we observed on a longer time scale by TEM (Figure ). To better understand the specificity of NaCl for isothermal growth, we set-up a protocol allowing isothermal assembly in the presence of Mg . Four tile strands were first thermally annealed in TAEMg, prior to introduction of the fifth strand (gray strand in Figure , time zero) and isothermal assembly at room temperature (Figure ). Representative microscopy images show the growth of the tubes over time (Figure ). The average length of nanotubes in TAEMg plateaus after around 30 minutes, in striking contrast to the growth in TANa buffer. The early saturation was also evident from both the frequency-based and length fraction histograms (Figure ).