The pure conduction state of a horizontal layer of fluid heated from below
becomes unstable with respect to a convecting state when the temperature difference exceeds a critical value. We examine the question of how real, physical
systems evolve from conduction to convection. Most experimental cells contain
geometric or thermal inhomogeneities which render the bifurcation to convection imperfect. In that case the pure conduction state never exists and the conveering state evolves continuously and smoothly as the temperature difference is
raised. When a sufficiently perfect experimental cell is constructed to eliminate
this route to convection, then dynamic imperfections will usually prevail. When
the temperature difference across the cell is raised, the vertical gradients in the
sidewalls evolve at a rate which differs from that in the fluid. The resulting
transient horizontal thermal gradients initiate the convective flow. This
phenomenon can be eliminated by providing sidewalls which have the same
thermal diffusivity as that of the fluid. When that is done, the convective flow is
started by random noise which exists in any experimental system. Analysis of
experiments shows that the noise source is considerably stronger than thermal
noise, but its origin is unclear at this time.