In Figure , constrictions 1 and 2 determine the flow rate. The angle at 3 controls vortex (and hence zoospore) injection into the stub, 4, whose length controls the number of vortices within the stub and the fluid velocity within them. The gradient of chemotactic attractant within the stub induces zoospores to move across the flow streamlines towards and into the detection channel, 5, whose length determines the attractant flux and hence gradient within the stub, 4.