  Organisms show a remarkable range of sizes, yet the dimensions of a single
cell rarely exceed $100$ $\mu$m. While the physical and biological origins of
this constraint remain poorly understood, exceptions to this rule give valuable
insights. A well-known counterexample is the aquatic plant $Chara$, whose cells
can exceed $10$ cm in length and $1$ mm in diameter. Two spiraling bands of
molecular motors at the cell periphery drive the cellular fluid up and down at
speeds up to $100$ $\mu$m/s, motion that has been hypothesized to mitigate the
slowness of metabolite transport on these scales and to aid in homeostasis.
This is the most organized instance of a broad class of continuous motions
known as "cytoplasmic streaming", found in a wide range of eukaryotic organisms
- algae, plants, amoebae, nematodes, and flies - often in unusually large
cells. In this overview of the physics of this phenomenon, we examine the
interplay between streaming, transport and cell size, and discuss the possible
role of self-organization phenomena in establishing the observed patterns of
streaming.
