  We suggest that molecular cloud (MC) turbulence is a consequence of the very
process of MC formation by collisions of larger-scale flows in the diffuse
atomic gas, which generate turbulence in the accumulated gas through bending-
mode instabilities. Turbulence is thus maintained for as long as the
accumulation process lasts ($\sim$ several Myr). Assuming that supersonic
turbulence in MCs has the double role of preventing global collapse while
promoting the formation of smaller-scale structures by turbulent compression
(``turbulent fragmentation''), we then note the following properties: a)
Turbulent fragmentation necessarily deposits progressively smaller fractions of
the total mass in regions of progressively smaller sizes, because the smaller
structures are subsets of the larger ones. b) The turbulent spectrum implies
that smaller scales have smaller velocity differences. Therefore, below some
scale, denoted $l_{\rm eq}$, the turbulent motions become subsonic. This is an
energy {\it distribution} phenomenon, not a dissipative one. On this basis, we
propose that the star formation efficiency (SFE) is determined by the fraction
of the total mass that is deposited in clumps with masses larger than $M_{\rm
J}(l_{\rm eq})$, the Jeans mass at scale $l_{\rm eq}$, because subsonic
turbulence cannot promote any further subfragmentation. In this scenario, the
SFE should be a monotonically increasing function of the sonic and turbulent
equality scale, $l_{\rm eq}$. We present preliminary numerical tests supporting
this prediction, and thus the suggestion that (one of the) relevant
parameter(s) is $l_{\rm eq}$, and compare with previous proposals that the
relevant parameter is the energy injection scale.
