Rod-shaped cells have a useful property: they can grow by extending their length without significantly changing the ratio of their surface area to volume, which would in turn change how efficiently they can take up nutrients from the environment. This property enables an impressive range of lengths for rod-shaped cells, from the short Cupriavidus necator you just saw, to this much longer Hylemonella gracilis.
The length of a cell (or, more generally, its size) varies depending on the environment or its stage of the lifecycle, but it does not vary much. Size tends to be strongly conserved within a species, ranging not much more than the factor of two dictated by replication. Sizes between species vary much more widely, as you will see throughout this book. Keep in mind, too, that the species we are able to image directly by cryo-ET are relatively small. Other species can be much larger, in some exceptional cases up to 100 μm across, visible to the naked eye.
